Check if the first character is '\0'. You should also probably check if your pointer is NULL.

char *c = "";
if ((c != NULL) && (c[0] == '\0')) {
   printf("c is empty\n");
}

You could put both of those checks in a function to make it convenient and easy to reuse.

Edit: In the if statement can be read like this, "If c is not zero and the first character of character array 'c' is not '\0' or zero, then...".

The && simply combines the two conditions. It is basically like saying this:

if (c != NULL) { /* AND (or &&) */
    if (c[0] == '\0') {
        printf("c is empty\n");
    }
}

You may want to get a good C programming book if that is not clear to you. I could recommend a book called "The C Programming Language".

The shortest version equivalent to the above would be:

if (c && !c[0]) {
  printf("c is empty\n");
}

Since C/C++ pointers can be interpreted as arrays of values the pointers point to, the two ways of checking values of a char* is by applying an indexing operator or by using pointer arithmetics. You can do this:

const char *p = myFunctionReturningConstChar();
for (int i = 0 ; p[i] ; i++) {
    if (p[i] == '\n') printf("New line\n");
}

or this:

const char *p = myFunctionReturningConstChar();
while (*p) {
    if (*p == '\n') printf("New line\n");
    p++;
}

In addition, C++ library provides multiple functions for working with C strings. You may find strlen helpful to check if your pointer points to an empty string.

You didn't initialize test. If you want it to be NULL initially, you have to set it:

const char* test = NULL;

If you want to check if the string is empty, the most direct way would be:

if (line[0] == '\0')
  continue;

However, it is easier to use a C++ string type, like std::string_view, where you can just use ==:

std::string_view line = repl.input("> ");
if (line == "")
   continue;