SendGrid: force empty text and html to " " with template_id

Work around an unexpected limitation in SendGrid template
rendering, where template text or html bodies are omitted
if the supplied message text or html is "". Changing empty
string to " " works around the issue.

https://sendgrid.com/docs/API_Reference/Web_API_v3/Transactional_Templates/smtpapi.html#-Text-or-HTML-Templates

Closes #32

tests/test_sendgrid_backend.py

@@ -400,6 +400,7 @@ class SendGridBackendAnymailFeatureTests(SendGridBackendMockAPITestCase):
         self.assertEqual(smtpapi['filters']['opentrack'], {'settings': {'enable': 0}})
 
     def test_template_id(self):
+        self.message.attach_alternative("HTML Body", "text/html")
         self.message.template_id = "5997fcf6-2b9f-484d-acd5-7e9a99f0dc1f"
         self.message.send()
         smtpapi = self.get_smtpapi()
@@ -407,6 +408,19 @@ class SendGridBackendAnymailFeatureTests(SendGridBackendMockAPITestCase):
             'settings': {'enable': 1,
                          'template_id': "5997fcf6-2b9f-484d-acd5-7e9a99f0dc1f"}
         })
+        data = self.get_api_call_data()
+        self.assertEqual(data['text'], "Text Body")
+        self.assertEqual(data['html'], "HTML Body")
+
+    def test_template_id_with_empty_body(self):
+        # Text and html must be present (and non-empty-string), or the corresponding
+        # part will not render from the template. Make sure we fill in strings:
+        message = mail.EmailMessage(from_email='from@example.com', to=['to@example.com'])
+        message.template_id = "5997fcf6-2b9f-484d-acd5-7e9a99f0dc1f"
+        message.send()
+        data = self.get_api_call_data()
+        self.assertEqual(data['text'], " ")  # single space is sufficient
+        self.assertEqual(data['html'], " ")
 
     def test_merge_data(self):
         self.message.from_email = 'from@example.com'