Stop converting null string arguments into empty strings

[ChrisThrasher]

Some APIs check will convert null const char* arguments to an empty string. I propose CSFML stop doing this except where the API actually promises that the behavior of null arguments is defined.

For example: https://github.com/SFML/CSFML/blob/21d4571d5bb030f4eebd97786cdb85e6a4ec717e/src/CSFML/Network/Ftp.cpp#L224-L228