Fix sending of cancellations to uninvited guests

* Had to add an optional argument to the email creator for this case,
  since there may not be any remaining addressees to send to (if the
  only guest was uninvited)
* The optional argument forces the creation of a message, even if there
  are no addressees

Bug: 2548465
Change-Id: I88784f49ce7afedce331fc756c5ecef4e73a07bc
2 files changed