Prevent 500 is head repo does not have PullRequest unit in IsUserAllowedToUpdate...

Prevent 500 is head repo does not have PullRequest unit in IsUserAllowedToUpdate (#20839) (#20848)

Backport #20621

Some repositories do not have the PullRequest unit present in their configuration
and unfortunately the way that IsUserAllowedToUpdate currently works assumes
that this is an error instead of just returning false.

This PR simply swallows this error allowing the function to return false.

Fix #20621

Signed-off-by: Andrew Thornton <art27@cantab.net>
Signed-off-by: Andrew Thornton <art27@cantab.net>