Blank `TextField` and `LongTextField`

[piotrpospiech]

Closes #1332

• Deleting input from TextField and LongTextField sets undefined instead of empty string in the model
• This behavior is already implemented in other fields (playground with optional TextField and NumField)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.03%. Comparing base (e1c4707) to head (09e150d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1356 +/- ## ========================================== + Coverage 95.02% 95.03% +0.01% ========================================== Files 175 175 Lines 2892 2900 +8 Branches 763 777 +14 ========================================== + Hits 2748 2756 +8 Misses 59 59 Partials 85 85 ```

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