[BUGFIX] Always merge results from empty tabs

When rendering child elements of a tabs, not only
the rendered HTML is collected from these child elements,
but also hidden fields HTML, assets and a lot more.

Therefore it is crucial to merge the child results regardless
of whether the "html" property is empty.

Releases: master, 10.4, 9.5
Resolves: #91636
Relates: #89094
Change-Id: If169bd6486d3001466464462b29788b94fbb0943