To detect if a string is null or empty, you can use the following without including any external dependencies on your project and still keeping your code simple/clean:

if(myString==null || myString.isEmpty()){
    //do something
}

or if blank spaces need to be detected as well:

if(myString==null || myString.trim().isEmpty()){
    //do something
}

you could easily wrap these into utility methods to be more concise since these are very common checks to make:

public final class StringUtils{

    private StringUtils() { }   

    public static bool isNullOrEmpty(string s){
        if(s==null || s.isEmpty()){
            return true;
        }
        return false;
    }

    public static bool isNullOrWhiteSpace(string s){
        if(s==null || s.trim().isEmpty()){
            return true;
        }
        return false;
    }
}

and then call these methods via:

if(StringUtils.isNullOrEmpty(myString)){...}

and

if(StringUtils.isNullOrWhiteSpace(myString)){...}