Bug 11402: make Labels::_guide_box return undef if undefned data is passed

Bug 11402: make Labels::_guide_box return undef if undefned data is passed

That's it. A guide box cannot be created if invalid data is passed.

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script, includes new unit tests.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 4720cbc4145223f1ffd653efdefd1a4f724d3144)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>