I/6232: Table properties: default value == no value

[jodator]

Suggested merge commit message (convention)

Internal: Make default value empty in the model. Closes ckeditor/ckeditor5#6232.

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Additional information

• Done - should work. Code style might need improvements and there's a lot of code duplication but this might be a topic for later.

[coveralls]

Coverage remained the same at 100.0% when pulling c3a5fa8 on i/6232 into 97da473 on master.

[jodator]

AFAICS it fails only due to old friend:

Chrome 80.0.3987 (Linux 0.0.0) Table cell paragraph post-fixer should wrap in paragraph $text nodes placed directly in tableCell (on table cell modification) FAILED

[Reinmar]

All look good to me. Waitin for confirmation from @ckeditor/qa-team.

[Mgsy]

We've checked it and looks good 👍