Add a placeholder into an empty transclusion

If a template was setup without a default output and returns an empty result
to the transclusion, the user could not access the template dialog because
there was nowhere to hover over to get it visible. This commit checks the
output from the template after it was rendered with the given parameters
and if it returns empty, it adds a placeholder to the output so to give
the users way to access that template's transclusion dialog and edit
or remove it.

Bug: 55810
Change-Id: Ib842b401e74d79b6382cada6bb7c6048b713977c
Merge "Add a placeholder into an empty transclusion"