REST API: Correctly infer empty objects passed via query parameters.

Permit passing an empty object as the string "?obj=". The type of the passed empty argument is inferred from the registered schema.

Props TimothyBlynJacobs, steffanhalv, schlessera, dd32.
Fixes #42961.


git-svn-id: https://develop.svn.wordpress.org/trunk@47362 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -1250,6 +1250,10 @@ function rest_validate_value_from_schema( $value, $args, $param = '' ) {
 	}
 
 	if ( 'object' === $args['type'] ) {
+		if ( '' === $value ) {
+			$value = array();
+		}
+
 		if ( $value instanceof stdClass ) {
 			$value = (array) $value;
 		}