tdf#121384 don't leave a bare trailing : in PYTHONPATH

tdf#121384 don't leave a bare trailing : in PYTHONPATH

and don't insert any empty path entries if that situation
was to arise

Change-Id: I8d8183485f457c3e4385181fee07390c4bfef603
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/96707
Reviewed-by: Tomáš Chvátal <tchvatal@suse.com>
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
Tested-by: Jenkins
(cherry picked from commit b72705d5391b849fc70a0a4cac33523c0ea5d054)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/96803
Tested-by: Stephan Bergmann <sbergman@redhat.com>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Gbp-Pq: Name fix-PYTHONPATH.diff