[BUGFIX] Empty subpart template if no data given

substituteMarkerAndSubpartArrayRecursive() should return an empty string
instead of the raw subpart template if no data for that subpart is given.
To clear a subpart the data can be just an empty array.

Change-Id: Ia6dd92168a86058993648c2fe4768b2cc8436cde
Resolves: #47935
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/20505
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind