Correctly test QString for being empty

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
HTML export: fix packaging of theme dir

I was confused by the function name getSubsurfaceDataPath() - it does not
find paths relative to the "data" folder, if finds the path where we might
install folders like "data", "translations", or "theme".
"data" is for some reason where we install the "marbledata" files.
Therefore on both Mac and Windows we need to put the "theme" directory
next to the "data" directory, not below it.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>