For checking if a variable is falsey or if it has length attribute equal to zero (which for a string, means it is empty), I use:

function isEmpty(str) {
    return (!str || str.length === 0 );
}

(Note that strings aren't the only variables with a length attribute, arrays have them as well, for example.)

Alternativaly, you can use the (not so) newly optional chaining and arrow functions to simplify:

const isEmpty = (str) => (!str?.length);

It will check the length, returning undefined in case of a nullish value, without throwing an error. In the case of an empty value, zero is falsy and the result is still valid.

For checking if a variable is falsey or if the string only contains whitespace or is empty, I use:

function isBlank(str) {
    return (!str || /^\s*$/.test(str));
}

If you want, you can monkey-patch the String prototype like this:

String.prototype.isEmpty = function() {
    // This doesn't work the same way as the isEmpty function used 
    // in the first example, it will return true for strings containing only whitespace
    return (this.length === 0 || !this.trim());
};
console.log("example".isEmpty());

Note that monkey-patching built-in types are controversial, as it can break code that depends on the existing structure of built-in types, for whatever reason.

I don't think there's a real convention here; Element.innerHTML is a property that tests the given value to determine what to do. In Safari it behaves like this:

if (value === null || value === '') {
    // remove all contents
} else {
    // parse string representation of value into the elements contents
}

So both "" (empty string) and null are considered the same and the assignment will just remove all contents; I couldn't find conclusive evidence that would suggest other browsers work this way, but it seems very likely that it should be considered an implementation detail that you shouldn't rely upon (see update).

That said, the documented way of clearing an element is by assigning the empty string to this property.

Update

I've found this (inconclusive) email thread about the subject, highlighting that this behaviour is not standardised:

For .innerHTML = null Opera and Internet Explorer act as if the literal string "null" was used. Firefox acts as if "" was used.

The set of "truthy" and "falsey" values in JavaScript comes from the ToBoolean abstract operation defined in the ECMAScript spec, which is used when coercing a value to a boolean:

+--------------------------------------------------------------------------+
| Argument Type | Result                                                   |
|---------------+----------------------------------------------------------|
| Undefined     | false                                                    |
|---------------+----------------------------------------------------------|
| Null          | false                                                    |
|---------------+----------------------------------------------------------|
| Boolean       | The result equals the input argument (no conversion).    |
|---------------+----------------------------------------------------------|
| Number        | The result is false if the argument is +0, −0, or NaN;   |
|               | otherwise the result is true.                            |
|---------------+----------------------------------------------------------|
| String        | The result is false if the argument is the empty String  |
|               | (its length is zero); otherwise the result is true.      |
|---------------+----------------------------------------------------------|
| Object        | true                                                     |
+--------------------------------------------------------------------------+

From this table, we can see that null and undefined are both coerced to false in a boolean context. However, your fields.length === 0 does not map generally onto a false value. If fields.length is a string, then it will be treated as false (because a zero-length string is false), but if it is an object (including an array) it will coerce to true.

If fields should be a string, then !fields is a sufficient predicate. If fields is an array, your best check might be:

if (!fields || fields.length === 0)

There are just a few revisions I would make.

First, always use === instead of == in Javascript. You can read more about that on Stack Overflow.

Second, since undefined is mutable, I would reccomend using

typeof value === "undefined"

instead of

value === undefined

Third, I would remove the !value and value === "" conditions. They are redundant.

My Revision

I would use a slightly different approach than you:

String.isNullOrEmpty = function(value) {
  return !(typeof value === "string" && value.length > 0);
}

This checks if the type of the value is "string" (and thus non-null and not undefined), and if it is not empty. If so, it is not null or empty.

Note that this returns true for non-string inputs, which might not be what you want if you wanted to throw an error for an unexpected input type.

If you can replace null-s with empty strings on serialized string, do something like this:

data = JSON.parse(JSON.stringify(data).replace(/\:null/gi, "\:\"\"")); 

You could also use a function :

// "def" means "default (to undefined)"
function def(v) { if (v) return v; }
x = def(x);
y = def(y);

Well, you need at least two lines (cheating a bit) =D

This will check if the variable's type is number and if not it won't render anything:

typeof number === 'number' ? number : ''