Fix incorrect entity being used for missing indexes.

When reading non-existing entities after existing ones have been read, we should not re-use the previous context by accident. Instead a new blank entity should be used. This properly simulates the state of the missing entity, as it will be missing errors, and all attributes.

I also tried to add a test for #4792 but wasn't able to reproduce the error, so I'm waiting on some additional information.

Refs #4769