JsonProperty ValueError: No JSON object could be decoded

[jer-tx]

Not sure if this is intended behavior but, when migrating from ext.db to ndb, we have models that use a custom JSON property that ends up saving data as a TextProperty.

So, attempting to migrate to ndb and seeing there's a native JsonProperty, that seems like a good decision. In practice, empty values are returned as string.

https://github.com/GoogleCloudPlatform/datastore-ndb-python/blob/master/ndb/model.py#L1891-L1896

That ends up trying to essentially run json.loads('') which of course ends up with a ValueError

I've resorted to copying the JsonProperty from the model into my custom properties, and adjusting the _from_base_type method to have: if value == '': return None before the try block.

Curious if there's a better solution here, maybe the API can be updated to include this?