Fixed edge cases for clearOnHide with nested forms + new test cases.

[travist]

Link to Jira Ticket

https://formio.atlassian.net/browse/FIO-XXXX

Description

There was previously a file that would perform all the tests for clearOnHide within the "test/unit" folder. This file was unfortunately not being executed because it did not have the "unit.js" extension. Once I enabled this file, it indicated a few edge cases where the clearOnHide functionality was not working within nested forms.

This also highlighted a few performance issues with the existing checks being made, as well as some legacy code that was checking the isEmpty() for nested forms, which caused major performance issues as it would iterate through all the children to check their emptiness... this was only being added to fix a bug with regards to customDefaultValues in nested forms.

Breaking Changes / Backwards Compatibility

None

Dependencies

None

How has this PR been tested?

New automated tests added.

Checklist:

• I have completed the above PR template
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• My changes include tests that prove my fix is effective (or that my feature works as intended)
• New and existing unit/integration tests pass locally with my changes
• Any dependent changes have corresponding PRs that are listed above