ChangeJson#getApprovalInfo: Add @Nullable annotations

The permittedVotingRange and tag parameters may be null, so
annotate them with @Nullable.

Change-Id: I6698ca34f512c783fbf319f63222804e6aec2586

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

@@ -994,8 +994,8 @@ public class ChangeJson {
   public static ApprovalInfo getApprovalInfo(
       Account.Id id,
       Integer value,
-      VotingRangeInfo permittedVotingRange,
-      String tag,
+      @Nullable VotingRangeInfo permittedVotingRange,
+      @Nullable String tag,
       Timestamp date) {
     ApprovalInfo ai = new ApprovalInfo(id.get());
     ai.value = value;