Avoid inefficient way to call functions without argument

Don't pass "()" format to PyObject_CallXXX() to call a function without
argument: pass NULL as the format string instead. It avoids to have to parse a
string to produce 0 argument.

Python/modsupport.c

@@ -468,7 +468,7 @@ va_build_value(const char *format, va_list va, int flags)
     int n = countformat(f, '\0');
     va_list lva;
 
-        Py_VA_COPY(lva, va);
+    Py_VA_COPY(lva, va);
 
     if (n < 0)
         return NULL;