FPHSA: Fix regression when VERSION_VAR is missing

If a package is found but FPHSA is called by the find module without a
`VERSION_VAR`, and the `find_package` call specifies a version, we have
previously accepted the package as found.  This was accidentally
regressed by commit 8f50f135ae (FPHSA: Improve error message when
VERSION_VAR is empty or has been unset(), 2022-08-01).  Restore it and
add a test case.