Issue #2721: Add string for FxA profile screen when no display name

[dnarcese]

Checklist

• Confirm the acceptance criteria is fully satisfied in the issue(s) this PR will close
• Add thorough tests or an explanation of why it does not
• Add a CHANGELOG entry if applicable
• Add QA labels on the associated issue (not this PR; qa-ready or qa-notneeded)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c29a62c). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #2853   +/-   ##
========================================
  Coverage          ?   51.5%           
  Complexity        ?     471           
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  Files             ?     121           
  Lines             ?    3524           
  Branches          ?     456           
========================================
  Hits              ?    1815           
  Misses            ?    1558           
  Partials          ?     151

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