Ensure that SMTP message body is not empty

Email notifications of both the start of a deploy, and of a failed
deploy, have empty message bodies. Azure Communication Services rejects
emails with empty message bodies with:

> smtplib.SMTPDataError: (501, b'5.6.0 Request body validation error. Need either non-empty Html or PlainText body to be present.')

src/commcare_cloud/commands/deploy/utils.py

@@ -100,14 +100,19 @@ def send_email(environment, subject, message='', to_admins=True, recipients=None
     """
     Call a Django management command to send an email.
 
-    :param environment: The Environement object
+    :param environment: The Environment object
     :param subject: Email subject
     :param message: Email message
     :param to_admins: True if mail should be sent to Django admins
     :param recipients: List of additional addresses to send mail to
     """
     if environment.fab_settings_config.email_enabled:
         print(color_summary(f">> Sending email: {subject}"))
+
+        if not message:
+            # Azure Communication Services require a message body
+            message = subject
+
         args = [
             message,
             '--subject', subject,