Fix ReturnMissingNullable bug pattern flagged by error prone

For more details see: [1].

[1] https://errorprone.info/bugpattern/ReturnMissingNullable
Change-Id: Ice845315af628157b805792a90e38728a8c0d747

src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java
index acc2ead..1f8826f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java
@@ -15,6 +15,7 @@
 package com.googlesource.gerrit.plugins.reviewnotes;
 
 import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.extensions.events.GitReferenceUpdatedListener;
@@ -69,6 +70,7 @@
           }
 
           @Override
+          @Nullable
           public String getRemoteName() {
             return null;
           }