crashfix: SendUserEmailOnFieldUpdate crashes on users without an email

Created by: efritz

log15.Log methods have the signature that expects a message, key, value, key, value, .... Here, we're passing ctx where it expects a string, which causes the log line to crash the current goroutine.

This was preventing all e2e tests from running.

Fixes https://github.com/sourcegraph/sourcegraph/issues/14649