Fix use of preprocessor null directive

This was broken recently in commit 255c929c56
for issue #217

Source/Preprocessor/cpp.c

@@ -1768,6 +1768,8 @@ String *Preprocessor_parse(String *s) {
 	}
       } else if (Equal(id, kpp_level)) {
 	Swig_error(Getfile(s), Getline(id), "cpp debug: level = %d, startlevel = %d\n", level, start_level);
+      } else if (Equal(id, "")) {
+	/* Null directive */
       } else {
 	Swig_error(Getfile(s), Getline(id), "Unknown SWIG preprocessor directive: %s\n", id);
       }