Fix broken data for time-zones with no transitions

While an invalid time-zone shall have no transitions, so may various
constant zones, like UTC.  The TZ data may include only the POSIX rule
for such a zone, in which case we should use it, even if there are no
transitions.

Broke out a piece of repeated code as a common method, in the process,
since I was complicating it further.

Added test for the case that revealed this; and made sure we see a
warning if any of the checkOffset() tests gets skipped because its
zone is unsupported.

Fixes: QTBUG-74614
Change-Id: Ic8e039a2a9b3f4e0f567585682a94f4b494b558d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions