[BUG] Allow getting SMS template from file instead of database

Without this change it's impossible to use a template file for communication event type SMS templates.

In ba59c014 the test that decides if the model field or a template file is
used for the SMS template was changed to is not None. However,
sms_template, unlike the other three template fields, does not allow NULL
values, so this test would always succeed, and thus a template file would
never be used for the SMS.