Handle edge case when padding is None in _migrate_deprecated_padding_xy

[JacekJagosz]

Fixes https://github.com/kivy/kivy/issues/8280

Maintainer merge checklist

• [ ] Title is descriptive/clear for inclusion in release notes.
• [ ] Applied a Component: xxx label.
• [ ] Applied the api-deprecation or api-break label.
• [ ] Applied the release-highlight label to be highlighted in release notes.
• [ ] Added to the milestone version it was merged into.
• [ ] Unittests are included in PR.
• [ ] Properly documented, including versionadded, versionchanged as needed.

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[JacekJagosz]

This is a regression with Kivy 2.2 from 2.1, where some code using KivyMD will fail with very indescriptive error. This makes the code work just as it did with 2.1.