Property editor validation and nullable argument fixes

[elliette]

This PR fixes a few issues in the Property Editor that are all semi-related:

1. Validation error "Please enter a string" when editing strings: [Property Editor] "Please enter a string error" #8764
2. Can't set numeric input to null: [property editor] type mismatch for int field #8714
3. Can't set optional nullable value to null: [Property Editor] Should be able to set optional nullable value to null #8757
4. Deleting value in nullable input sets value to null: [property editor] removing value from a non-required field should return to null or default value #8709

Work towards #1948
Fixes #8764, #8714, #8757, #8709