[9.x] Use json_validate (#673)

Author: duncanmcclean

src/Http/Controllers/CP/Traits/PreparesModels.php

@@ -47,7 +47,7 @@ protected function prepareModelForPublishForm(Resource $resource, Model $model):
                 }
 
                 // When $value is a JSON string, we need to decode it.
-                if (Json::isJson($value)) {
+                if (is_string($value) && json_validate($value)) {
                     $value = json_decode((string) $value, true);
                 }