fix: check potential variables for flyout variable fields

[maribethb]

The basics

• I validated my changes

The details

Resolves

Fixes #8863

Proposed Changes

• Fixes the bug where variable fields in flyouts weren't allowing setting their types correctly
• Adds test for the above case
• Fixes a test for the case where variableTypes was undefined in the constructor. The old test comments were incorrect and the test only passed because the test case block did not have a sourceBlock property. The correct behavior is that passing undefined behaves the same as passing null.
• Updated the type signature and tsdoc to explictly allow passing null since that's what we say to do if you want to see all the variable types.
• Moved the code checking that you don't set an empty array to the setTypes method with the rest of the validation instead of the getTypes method.
• Removes @internal from workspace.getPotentialVariableMap since it is called from FieldVariable which may be subclassed.

Reason for Changes

• Fixing bugs and improving tests

Test Coverage

Added tests and tested manually using the test block from the issue description

Documentation

Additional Information

[rachel-fenichel]

Move this comment to after this if, so that it's closer to the code it describes.