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.

Refs #4769
Merge pull request #4803 from cakephp/issue-form-helper-error

Fix incorrect entity being used for missing indexes.