Fix (table): Support cells without children

[mat128]

Previously, it was possible for table cells to be entirely empty.
However, a recent change introduced in 93dbbf9 changed the evaluation order for table cell children.

This led to calling hasAnyAttribute on a null reference, which is not supported.

Move isSingleParagraph check prior to hasAnyAttribute. isSingleParagraph is correctly calculated because it relies on the childCount for the table cell.

Fixes #8941
Fixes #8917

[testinfected]

+1

[bsmilling]

+1, would be nice if this was included in the next version.

[oleq]

Hi, @mat128! 

Thank you for the contribution, your PR totally makes sense. There's one thing missing, though: a unit test for the change. Could you provide it for us to avoid future regressions? Thanks.

[wenneberg6]

+1 this bug halted us from upgrading to v. 25

[mat128]

@oleq Thank you for your feedback. I tried writing a unit test but could never trigger the problem.

It appears empty cells have a   added when invoking utility functions for data/model conversion.

it( 'should support empty cells', () => {
	setModelData( model, modelTable( [ [ '' ] ] ) );
	assertEqualMarkup( editor.getData(),
		'<figure class="table">' +
			'<table>' +
				'<tbody>' +
					'<tr><td>&nbsp;</td></tr>' +
				'</tbody>' +
			'</table>' +
		'</figure>'
	);
} );

I also tried setting model data directly, to no avail:

setModelData(model, '<table><tableRow><tableCell></tableCell></tableRow></table>');

Could point me on the right track?

Thank you

[dkrahn]

Probably also fixes #8979

[oleq]

@mat128 Don't worry, we took it over and finished this task in #9004. Thanks again!

[mat128]

Thanks for the quick turnaround! Good job =)