[errorprone] Annotate method returning null as nullable

See https://errorprone.info/bugpattern/ReturnMissingNullable

Change-Id: Ic1d29e357c6305df28760d92080c2a546675a843

java/com/google/gitiles/TreeSoyData.java

diff --git a/java/com/google/gitiles/TreeSoyData.java b/java/com/google/gitiles/TreeSoyData.java
index 985edfa..4e2a579 100644
--- a/java/com/google/gitiles/TreeSoyData.java
+++ b/java/com/google/gitiles/TreeSoyData.java
@@ -26,6 +26,7 @@
 import java.io.IOException;
 import java.util.List;
 import java.util.Map;
+import javax.annotation.Nullable;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.ObjectId;
@@ -55,7 +56,7 @@
    */
   static final int MAX_SYMLINK_SIZE = 16 << 10;
 
-  static String resolveTargetUrl(GitilesView view, String target) {
+  static @Nullable String resolveTargetUrl(GitilesView view, String target) {
     String resolved = PathUtil.simplifyPathUpToRoot(target, view.getPathPart());
     if (resolved == null) {
       return null;