Importing non-descriptor based field mapping, then exporting with mapping that requires it exports 'null' desc

[Col-E]

Importing SRG mappings and exporting as Simple mappings results in invalid output because of inclusions of null as field descriptors.

Exporting as TinyV1 seems to work fine. The fix for simple export would be to not include the null desc string since simple allows for skipping field descriptors when reading. Seems to be an issue where the field key for MappingsAdapter emits null as a field desc during the export process because the export mappings (Simple) supports field type differentiation, but the input (SRG) does not.