Missing data in one subfield excludes whole field from export with the custom mapping profile

Overview:
When exporting inventory records with a custom mapping profile and one of the mapped subfields is not populated in the inventory record then the whole MARC field is omitted from export
Steps to Reproduce:
1. Create a custom mapping profile with following transformations:

2. Create a job profile linked to mapping profile from the previous step
3. Save UUID of a record that have a permanent and effective locations populated but does not have Volume in the item record populated (for example Nod in the snapshot environment)
4. Export record using the job profile from the step 2

Expected Results:
Exported file contains 866 and 946 fields populated with the available data. For example:
=866 \0$aLine 1b$zMain Library
=866 \0$aLine 2b$zMain Library
=946 \0$zMain Library$3hold000000000008

Actual Results:
946 field is missing.

Additional Information:
Populating volume information (even setting it to space) on the item record resolve the problem.
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