task(admin): Fix bug when diagnosticCode is null

[dschom]

Because

• Diagnostic code can be null in the database
• We recently fixed a different bug, where inserts were failing because were providing undefined instead null, hence null values are now being written.

This pull request

• Fixes the schema to support nullable values.

Issue that this pull request solves

Closes: FXA-127896

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• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

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