fix: not being able to set field values to empty

[BeksOmega]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

Fixes #6701

Proposed Changes

Makes it so that you can set a text field to empty.

Reason for Changes

Sometimes the empty string is useful!

Test Coverage

Mostly manual testing. Also updated some change-detector tests to not fail. (gah past self)

Documentation

N/A

Additional Information

I think the issue started here: #6073

Basically, we used to be checking the text value against null and then we were checking it against '' (to see if it had changed), which was failing.

So now, instead of trying to do this weird thing to check if the value had changed, just let setValue handle it like a normal field, and set isDirty_ in the doValueUpdate_ and doValueInvalid_ functions instead of forcing a layout "manually".