return uninitialized arrays as []

The methods "concepts", "term_groups", and "lang_groups" return array
pointers containing other objects. James Hayes found that if a given
array property had never been initialized with anything, the method
would return "undef" isntead of "[]". Fix this bug and provide
regression tests.
satisfy perlcritic

End _date_created in a return statement to satisfy perlcritic before
the next release.
New CPAN release of TBX-Min - v0.03

    Uninitialized arrays are now returned as [] instead of undef