feat: permit nullable parent

[solanamonk]

At the moment, if there is a record whose entityType and entityId are null, you get an exception when it gets hydrated:

EntityMetadataNotFoundError: No metadata for "null" was found.

This pull request makes it possible to have a nullable polymorphic parent.

[bashleigh]

Yea this looks good, good shout! Never thought of that being a requirement

[jromerof]

@bashleigh is this going to be published? 🙏 I think the last npm release was over a year ago