py/mpprint: Only check for null string printing when NDEBUG not defined.

Printing "(null)" when a NULL string pointer is passed to %s is a debugging
feature and not a feature that's relied upon by the code.  So it only needs
to be compiled in when debugging (such as assert) is enabled, and saves
roughy 30 bytes of code when disabled.

This patch also fixes this NULL check to not do the check if the precision
is specified as zero.

py/mpprint.c

@@ -485,14 +485,17 @@ int mp_vprintf(const mp_print_t *print, const char *fmt, va_list args) {
             case 's':
             {
                 const char *str = va_arg(args, const char*);
-                if (str) {
+                #ifndef NDEBUG
+                // With debugging enabled, catch printing of null string pointers
+                if (prec != 0 && str == NULL) {
+                    chrs += mp_print_strn(print, "(null)", 6, flags, fill, width);
+                    break;
+                }
+                #endif
                 if (prec < 0) {
                     prec = strlen(str);
                 }
                 chrs += mp_print_strn(print, str, prec, flags, fill, width);
-                } else {
-                    chrs += mp_print_strn(print, "(null)", 6, flags, fill, width);
-                }
                 break;
             }
             case 'u':