fix(migrations): support --defaults in standalone migration (#48921)

Fixes that running the standalone migration with `--defaults` was throwing an error, because all the properties were specified as required. We don't need to set them as required, because they have default values.

Fixes #48845.

PR Close #48921

packages/core/schematics/ng-generate/standalone-migration/schema.json

@@ -30,13 +30,9 @@
     },
     "path": {
       "type": "string",
-      "$default": {
-        "$source": "workingDirectory"
-      },
       "description": "Path relative to the project root which should be migrated",
       "x-prompt": "Which path in your project should be migrated?",
       "default": "./"
     }
-  },
-  "required": ["mode", "path"]
+  }
 }