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
(cherry picked from commit d4659620aa109a9d9ea6641d18e9e473117cd22d)

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 03bff09..b83f1c6 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
@@ -116,7 +116,9 @@
   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()));
     return issueExtractor.getIssueIds(event.getProjectNameKey().get(),
         event.refUpdate.get().newRev);