MGCP TRD 4.5.1 default values - NFI and NFN

[tdvorakcaci]

Describe the bug Field values set to N_A for NFI and NFN at data import with Hootenanny, are exported with field values of UNK. N_A is the correct default value for blank NFI and NFN attribution.

To Reproduce Steps to reproduce the behavior:

1. Import MGCP TRD v4.5.1 data with Hootenanny
2. Export data with MGCP schema switcher with Hootenanny

Expected behavior NFI and NFN values should remain N_A on Hootenanny import and export.

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Additional context Add any other context about the problem here.

[mattjdnv]

Fixed with other schema changes in #5216