Annotate methods that return a definitely null value with @Nullable

We intend to enable the ReturnMissingNullable errorprone pattern in
Gerrit core and this fixes the current issues.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I4bd1ff5d6cce8c1c4babe8cbd9f2266365e63987

src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java b/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java
index f2f842c..1ff6af0 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java
@@ -17,6 +17,7 @@
 import static java.util.stream.Collectors.toMap;
 
 import com.google.common.collect.ImmutableList;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.server.CurrentUser;
@@ -166,6 +167,7 @@
     throw new UnsupportedOperationException(); // Gitiles is read-only.
   }
 
+  @Nullable
   @Override
   public ReflogReader getReflogReader(String refName) throws IOException {
     return exactRef(refName) != null ? delegate.getReflogReader(refName) : null;
@@ -210,6 +212,7 @@
       throw new UnsupportedOperationException(); // Gitiles is read-only.
     }
 
+    @Nullable
     @Override
     public Ref exactRef(String name) throws IOException {
       Ref ref = git.getRefDatabase().exactRef(name);