slack_incoming: Handle explicit None text payload in webhook.

zerver/webhooks/slack_incoming/fixtures/null_text.json

@@ -0,0 +1,3 @@
+{
+    "text": null
+}

zerver/webhooks/slack_incoming/tests.py

@@ -18,6 +18,12 @@ Hello, world.
             expected_message,
         )
 
+    def test_null_message(self) -> None:
+        self.check_webhook(
+            "null_text",
+            expect_noop=True,
+        )
+
     def test_message_as_www_urlencoded(self) -> None:
         expected_topic = "devops"
         expected_message = """

zerver/webhooks/slack_incoming/view.py

@@ -50,7 +50,7 @@ def api_slack_incoming_webhook(
         for attachment in payload["attachments"]:
             body = add_attachment(attachment, body)
 
-    if body == "" and "text" in payload:
+    if body == "" and "text" in payload and payload["text"] is not None:
         body += payload["text"]
         if "icon_emoji" in payload and payload["icon_emoji"] is not None:
             body = "{} {}".format(payload["icon_emoji"], body)