Avoid exception in case of non-existing user
Avoid exception in case of non-existing user

A non-existing user can authenticate with the policy
passOnNoUser. But if also a notification event is
defined, this would result in an exception, that the
user does not exist.

So we sanitize the user in the eventhandler module.

Closes #634

wheldom01/privacyidea,jh23453/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,privacyidea/privacyidea,jh23453/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,jh23453/privacyidea,privacyidea/privacyidea,jh23453/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,jh23453/privacyidea,jh23453/privacyidea,privacyidea/privacyidea