Avoid NPE when ChangeData.commitFooters() returns null

Change-Id: I02d1e43dfa507e16c51dab6b27005fe5760ab93b

java/com/google/gerrit/server/config/TrackingFooters.java

@@ -37,9 +37,6 @@ public class TrackingFooters {
 
   public ListMultimap<String, String> extract(List<FooterLine> lines) {
     ListMultimap<String, String> r = MultimapBuilder.hashKeys().arrayListValues().build();
-    if (lines == null) {
-      return r;
-    }
 
     for (FooterLine footer : lines) {
       for (TrackingFooter config : trackingFooters) {

java/com/google/gerrit/server/query/change/ChangeData.java

@@ -543,10 +543,11 @@ public class ChangeData {
     return commitMessage;
   }
 
+  /** Returns the list of commit footers (which may be empty). */
   public List<FooterLine> commitFooters() {
     if (commitFooters == null) {
       if (!loadCommitData()) {
-        return null;
+        return ImmutableList.of();
       }
     }
     return commitFooters;