Add @Nullable on SubscriptionInfo#getGroupUuid

SubscriptionInfo#getGroupUuid will return null if group is not set.

Bug: 123587615
Test: build
Change-Id: I592d35388d88bda1f908441824772fea36d43059

api/current.txt

@@ -45016,7 +45016,7 @@ package android.telephony {
     method public String getCountryIso();
     method public int getDataRoaming();
     method public CharSequence getDisplayName();
-    method public String getGroupUuid();
+    method @Nullable public String getGroupUuid();
     method public String getIccId();
     method public int getIconTint();
     method @Deprecated public int getMcc();

telephony/java/android/telephony/SubscriptionInfo.java

@@ -467,7 +467,7 @@ public class SubscriptionInfo implements Parcelable {
      * @return group UUID a String of group UUID if it belongs to a group. Otherwise
      * it will return null.
      */
-    public String getGroupUuid() {
+    public @Nullable String getGroupUuid() {
         return mGroupUUID;
     }