IncludedIn: Don't fail with NPE if ExternalIncludedIn returns null

Change-Id: I1daf70677e58dbde22ab65f081c5e3eb5d1fd401
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/change/IncludedIn.java

diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/IncludedIn.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/IncludedIn.java
index 404fe75..344cb44 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/IncludedIn.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/IncludedIn.java
@@ -83,8 +83,11 @@
       IncludedInResolver.Result d = IncludedInResolver.resolve(r, rw, rev);
       Multimap<String, String> external = ArrayListMultimap.create();
       for (ExternalIncludedIn ext : includedIn) {
-        external.putAll(ext.getIncludedIn(project.get(), rev.name(),
-            d.getTags(), d.getBranches()));
+        Multimap<String, String> extIncludedIns = ext.getIncludedIn(
+            project.get(), rev.name(), d.getTags(), d.getBranches());
+        if (extIncludedIns != null) {
+          external.putAll(extIncludedIns);
+        }
       }
       return new IncludedInInfo(d,
           (!external.isEmpty() ? external.asMap() : null));