Cleanup: add NULL check for filename argument which could be NULL

While it's never NULL at the moment, checks elsewhere in this function
support passing in a NULL filename, so keep this working as intended
in case RNAProcessItem.filename is NULL in the future.

source/blender/makesrna/intern/makesrna.c

@@ -4436,7 +4436,7 @@ static void rna_generate(BlenderRNA *brna, FILE *f, const char *filename, const
     }
   }
 
-  if (STREQ(filename, "rna_ID.c")) {
+  if (filename && STREQ(filename, "rna_ID.c")) {
     /* this is ugly, but we cannot have c files compiled for both
      * makesrna and blender with some build systems at the moment */
     fprintf(f, "#include \"rna_define.c\"\n\n");