FC_GetHeight of empty string= 0

[MonzUn]

Previous to this commit FC_GetHeight returned the line height if it was supplied with an empty string. It will now return 0 in this case.

If you prefer to check against empty string in some other way I can change it. I mostly work with C++ so I'm not sure if this is the go-to way of checking against empty string in C.

I also ran the test and it was unaffected.