strstr(3): add special case for empty needle, as in wcsstr(3)

Hi,

in the section return value, wcsstr(3) provides the following note
for the special case of an empty needle:

<https://man7.org/linux/man-pages/man3/wcsstr.3.html#RETURN_VALUE>

|  Note the special case: If needle is the empty wide-character
|  string, the return value is always haystack itself.

The same note is but missing in strstr(3) and should be added there:

<https://man7.org/linux/man-pages/man3/strstr.3.html#RETURN_VALUE>

|  Note the special case: If needle is the empty string, the return
|  value is always haystack itself.

regards
Stefan