Show the user's email for external invitations in the action dialogs

[joepavitt]

Description

Fix an undefined error when the invite is for an external user, introduced in #5151

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.28%. Comparing base (71709cf) to head (5c7bccb).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5158   +/-   ##
=======================================
  Coverage   78.28%   78.28%           
=======================================
  Files         339      339           
  Lines       16000    16000           
  Branches     3702     3702           
=======================================
  Hits        12526    12526           
  Misses       3474     3474           

Flag	Coverage Δ
backend	78.28% <ø> (ø)

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