fix(webhooks): Test for empty ref on Git hooks (#590)

* fix(webhooks):  Test for empty ref on Git hooks

Some Git webhooks don't have a "ref" field, which is assumed in this
handler.  Sets the branch to empty string in this case (presuming other
code is expecting to find a non-null value further downstream).

echo-webhooks/src/main/groovy/com/netflix/spinnaker/echo/scm/GithubWebhookEventHandler.java

@@ -63,7 +63,11 @@ public void handle(Event event, Map postedEvent) {
         objectMapper.convertValue(postedEvent, GithubWebhookEvent.class);
 
     event.content.put("hash", githubWebhookEvent.after);
-    event.content.put("branch", githubWebhookEvent.ref.replace("refs/heads/", ""));
+    if (githubWebhookEvent.ref == null) {
+      event.content.put("branch", "");
+    } else {
+      event.content.put("branch", githubWebhookEvent.ref.replace("refs/heads/", ""));
+    }
     event.content.put("repoProject", githubWebhookEvent.repository.owner.name);
     event.content.put("slug", githubWebhookEvent.repository.name);
   }

echo-webhooks/src/test/groovy/com/netflix/spinnaker/echo/controllers/WebhooksControllerSpec.groovy

@@ -401,6 +401,42 @@ class WebhooksControllerSpec extends Specification {
     event.content.branch == "simple-tag"
   }
 
+  void 'handles non-push Github Webhook Event gracefully'() {
+    def event
+
+    given:
+    WebhooksController controller = new WebhooksController(mapper: new ObjectMapper(), scmWebhookHandler: scmWebhookHandler)
+    controller.propagator = Mock(EventPropagator)
+    controller.artifactExtractor = Mock(ArtifactExtractor)
+    controller.artifactExtractor.extractArtifacts(_, _, _) >> []
+
+    when:
+    def response = controller.forwardEvent(
+      "git",
+      "github",
+      """{
+          "before": "a10867b14bb761a232cd80139fbd4c0d33264240",
+          "after": "0000000000000000000000000000000000000000",
+          "repository": {
+            "name": "Hello-World",
+            "owner": {
+              "name": "Codertocat"
+            }
+          }
+        }
+        """, new HttpHeaders())
+
+    then:
+    1 * controller.propagator.processEvent(_) >> {
+      event = it[0]
+    }
+
+    event.content.hash == "0000000000000000000000000000000000000000"
+    event.content.repoProject == "Codertocat"
+    event.content.slug == "Hello-World"
+    event.content.branch == ""
+  }
+
   void "handles Gitlab Webhook Event"() {
     def event