Empty lines inside table cell cause empty WW result

[ibedarev]

Here is br tag causes stack.pop https://github.com/nhn/tui.editor/blob/0c5c11bac0b9954e2cac2ff2a081ce8c80a5f14d/apps/editor/src/convertors/toWysiwyg/htmlToWwConvertors.ts#L186

After convert cycle stack will be empty and converter will return null as result

Md example
1 1