deposit: thesis metadata fixes

* Fixes thesis metadata mapping when there is no data input.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
deposit: proceedings info box additions

* Adds nonpublic_note field (500__a) to
  Proceedings Info Box.

* Disables Proceedings Box if any journal
  information is filled.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Co-authored-by Georgios Kokosioulis <georgios.kokosioulis@cern.ch>
Co-authored-by Kamil Neczaj <kamil.neczaj@cern.ch>