slack_incoming: Handle null blocks and attachments.

This is not accepted according to Slack's block-builder, but is
attested in the wild.

zerver/webhooks/slack_incoming/fixtures/attachment.json

@@ -1,6 +1,7 @@
 {
     "channel": "#prometheus-alerts",
     "username": "Alertmanager",
+    "blocks": null,
     "attachments": [
         {
             "title": "[FIRING:2] InstanceDown for api-server (env=\"prod\", severity=\"critical\")",

zerver/webhooks/slack_incoming/view.py

@@ -50,11 +50,11 @@ def api_slack_incoming_webhook(
 
     body = ""
 
-    if "blocks" in payload:
+    if "blocks" in payload and payload["blocks"]:
         for block in payload["blocks"]:
             body = add_block(block, body)
 
-    if "attachments" in payload:
+    if "attachments" in payload and payload["attachments"]:
         for attachment in payload["attachments"]:
             body = add_attachment(attachment, body)