BibDocFile: safer new bibdocfile model upgrade

• FIX Safer upgrade recipe for migrations from the old document storage model (used in v1.1) to the new document storage model (used in v1.2).

• Uses the get() method instead of getitem() defaulting the return value to an empty dictionary in case of missing keys in more_info.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>