Do not allow nullable new_id

The usefulness of this is limited, and `libwayland-client` doesn't
provide a way to pass a null `new_id` since the id is generated by the
library and given to the caller as the return value.

Signed-off-by: Ian Douglas Scott <idscott@system76.com>

src/scanner.c

@@ -414,7 +414,6 @@ is_nullable_type(struct arg *arg)
 	/* Strings and objects are possibly nullable */
 	case STRING:
 	case OBJECT:
-	case NEW_ID:
 		return true;
 	default:
 		return false;