Fix GEDCOM import for SUBN record containing note

[prculley]

Fixes #13024, #12152

A note attached to a SUBN (submission) record was causing a crash if the use default source preference was not set.

While testing I also discovered that when "use default source" preference WAS set, the note never was attached to the source, but ended up as an unconnected object.

[emyoulation]

The GEDCOM Assessment testing results grid by John Cardinal have opportunities for improvement due to new features in Gramps 5.2

New in 5.2 is support for open-ended date spans. 10 samples were identified as "Imported incorrectly" in the Date Valid section. Can they be added? It would drop the count of "Imported incorrectly" from 18 to 8.
#1124

The Event Shared section indicates that the non-standard (but common) Role and Association data is not supported by the Gramps GEDCOM import. Since Roles and Associations have existed for a while in Gramps, can they be handled by the import? (Export of such non-standard tags would probably have to be elsewhere.)

References:
Cardinal, John. "GEDCOM Assessment v1.05" evaluating GEDCOM v5.5.1 import using Gramps AIO64-5.1.2-1 with the " assess.ged v1.05" test file, 7 Mar 2020 (GEDCOM Assessment change log