Fixed #27469 -- Prevented sending email to empty addresses

Thanks Jarek Glowacki for the report.

django/core/mail/message.py

@@ -328,7 +328,7 @@ class EmailMessage(object):
         Returns a list of all recipients of the email (includes direct
         addressees as well as Cc and Bcc entries).
         """
-        return self.to + self.cc + self.bcc
+        return [email for email in (self.to + self.cc + self.bcc) if email]
 
     def send(self, fail_silently=False):
         """Sends the email message."""

tests/mail/tests.py

@@ -100,6 +100,22 @@ class MailTests(HeadersCheckMixin, SimpleTestCase):
         self.assertEqual(message['From'], 'from@example.com')
         self.assertEqual(message['To'], 'to@example.com, other@example.com')
 
+    def test_recipients_with_empty_strings(self):
+        """
+        Empty strings in various recipient arguments are always stripped
+        off the final recipient list.
+        """
+        email = EmailMessage(
+            'Subject', 'Content', 'from@example.com', ['to@example.com', ''],
+            cc=['cc@example.com', ''],
+            bcc=['', 'bcc@example.com'],
+            reply_to=['', None],
+        )
+        self.assertEqual(
+            email.recipients(),
+            ['to@example.com', 'cc@example.com', 'bcc@example.com']
+        )
+
     def test_cc(self):
         """Regression test for #7722"""
         email = EmailMessage('Subject', 'Content', 'from@example.com', ['to@example.com'], cc=['cc@example.com'])