Don't crash with freetype supplies a non-null family name.

[jneem]

Fixes #5.

I don't know if returning an empty string is the right thing to do when FreeType has a null family name, but at least it doesn't crash. In any case, I've verified with the following C program that FreeType will happily load a font with no family name and report no errors:

#include <stdio.h>

#include <ft2build.h>
#include FT_FREETYPE_H

int main()
{
    FT_Library library;
    FT_Face face;

    FT_Init_FreeType(&library);
    FT_Error error = FT_New_Face(library, "/usr/share/fonts/misc/decsess.pcf.gz", 0, &face);
    if (error) {
        printf("Error opening face: %s\n", FT_Error_String(error));
        return 1;
    }
    printf("Success opening face. Family name: %s\n", face->family_name);
    return 0;
}

[RazrFalcon]

Thanks!