PropertyExtractor: Add null check for event.submitter

If the change was open, it would cause event.submitter to be
null. Fix this by adding a null check.

Gerrit's core does the same thing.

Bug: Issue 8051
Change-Id: I0690e184ba50ea1e60176e8be5af34c43a80393e

src/main/java/com/googlesource/gerrit/plugins/its/base/util/PropertyExtractor.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/its/base/util/PropertyExtractor.java b/src/main/java/com/googlesource/gerrit/plugins/its/base/util/PropertyExtractor.java
index b26db77..f9c589d 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/its/base/util/PropertyExtractor.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/its/base/util/PropertyExtractor.java
@@ -109,7 +109,9 @@
 
   private Map<String, Set<String>> extractFrom(RefUpdatedEvent event, Set<Property> common) {
     common.add(propertyFactory.create("event-type", event.type));
-    common.addAll(propertyAttributeExtractor.extractFrom(event.submitter.get(), "submitter"));
+    if (event.submitter != null) {
+      common.addAll(propertyAttributeExtractor.extractFrom(event.submitter.get(), "submitter"));
+    }
     common.addAll(propertyAttributeExtractor.extractFrom(event.refUpdate.get()));
     RefUpdateAttribute refUpdated = event.refUpdate.get();
     if (ObjectId.zeroId().name().equals(refUpdated.newRev)) {