bug(auth): Address case where sender is empty, null, or missing

[dschom]

Because

• We were seeing response validation errors in sentry
• The sender should be optional

This pull request

• Removes sender from response if empty, null, or undefined.

Issue that this pull request solves

Closes: FXA-12956

Checklist

Put an x in the boxes that apply

• 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).

Screenshots (Optional)

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Other information (Optional)

This state is a little bit bizarre. Typically the device id, which ends up being the sender value stored in the DB, is typically present on the DB write...

[vbudhram]

The test failures have been fixed on main, should be fixed after rebase.