Empty subtitles waste space on title slides
Even if no subtitle is set, an empty paragraph (containing a
\strut) with the required vertical skips before and after is created on title slides. This makes the
image variation of the title slide impossible to use with titles that span more than one line, because there is not enough room for a two line title, one (empty) subtitle line and the "Date | Author | Institute" line which in this case just drops out of the bottom of the "Banderole".
I have changes in https://gitlab.version.fz-juelich.de/bsteinb/beamertheme-juelich/tree/issue13 that do not create an empty paragraph if the subtitle is empty, but those build on the changes in the unmerged !3 (merged), so I am holding off on making a pull request for now.