make contentsDeltaUri optional (GoogleCloudPlatform#12637)

Author: yutatanamoto

mmv1/products/vertexai/Index.yaml

@@ -95,7 +95,6 @@ properties:
           Index field can be also updated as part of the same call.
           The expected structure and format of the files this URI points to is
           described at https://cloud.google.com/vertex-ai/docs/matching-engine/using-matching-engine#input-data-format
-        required: true
         custom_flatten: 'templates/terraform/custom_flatten/vertex_ai_index_ignore_contents_delta_uri.go.tmpl'
       - name: 'isCompleteOverwrite'
         type: Boolean