Make the "description" PM field in JSON optional
Make the "description" PM field in JSON optional

42dfe842 introduced reading of a "description" field from the PM entries from
the ASCII database for the needs of ScopeLoader. The ascii db files for large
barrel do not have this description though.
This commit makes the "description" field optional and sets it to an empty
string if description is not present in the ascii db.

alexkernphysiker/j-pet-framework,alexkernphysiker/j-pet-framework,JPETTomography/j-pet-framework,JPETTomography/j-pet-framework,JPETTomography/j-pet-framework,alexkernphysiker/j-pet-framework,alexkernphysiker/j-pet-framework,alexkernphysiker/j-pet-framework