Don't fire ray compatibility webhook when PR or branch is not provided (#36088)

Signed-off-by: Jeffrey Wang <jeffreywang@anyscale.com>

.buildkite/scripts/check-ray-compatibility.sh

@@ -166,8 +166,15 @@ See [issue #33599](https://github.com/vllm-project/vllm/issues/33599) for contex
 EOF
 fi
 
-# Notify Slack if webhook is configured.
+# Notify Slack if webhook is configured and PR/branch are valid.
 if [ -n "$RAY_COMPAT_SLACK_WEBHOOK_URL" ]; then
+    PR="${BUILDKITE_PULL_REQUEST:-}"
+    BRANCH="${BUILDKITE_BRANCH:-}"
+
+    # Skip notification if PR is invalid or branch is empty
+    if [[ "$PR" = "false" || -z "$PR" || -z "$BRANCH" ]]; then
+        echo ">>> Skipping Slack notification (invalid PR or empty branch: PR=$PR, branch=$BRANCH)"
+    else
         echo ">>> Sending Slack notification"
         # Single quotes are intentional: the f-string expressions are Python, not shell.
         # shellcheck disable=SC2016
@@ -198,6 +205,7 @@ print(json.dumps(data))
             -H 'Content-type: application/json' \
             -d "$PAYLOAD")
         echo "    Slack webhook response: $HTTP_CODE"
+    fi
 else
     echo ">>> Skipping Slack notification (RAY_COMPAT_SLACK_WEBHOOK_URL not set)"
 fi