Summary:
Choice/Reference List editor wasn't clearing itself when it received an empty string. It led
to a bug on the Card widget where pressing those keys resulted in the same behavior as
pressing Enter - it just opened the editor.
Grid view has it's own implementation for those keys, so it wasn't affected.
Test Plan: Added new test.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D3908
