fix RealmObjectObserver's callback is called repeatedly when the result is null.

realm-helpers/src/main/java/chat/rocket/android/realm_helper/RealmListObserver.java

@@ -30,7 +30,7 @@ public class RealmListObserver<T extends RealmObject> extends AbstractRealmResul
   @Override
   public final RealmChangeListener<RealmResults<T>> getListener() {
     return results -> {
-      String currentResultString = results != null ? results.toString() : null;
+      String currentResultString = results != null ? results.toString() : "null";
       if (previousResultsString != null && previousResultsString.equals(currentResultString)) {
         return;
       }

realm-helpers/src/main/java/chat/rocket/android/realm_helper/RealmObjectObserver.java

@@ -37,7 +37,7 @@ public class RealmObjectObserver<T extends RealmObject> extends AbstractRealmRes
   protected final RealmChangeListener<RealmResults<T>> getListener() {
     return element -> {
       T currentResult = impl.extractObjectFromResults(element);
-      String currentResultString = currentResult != null ? currentResult.toString() : null;
+      String currentResultString = currentResult != null ? currentResult.toString() : "null";
       if (previousResultString != null && previousResultString.equals(currentResultString)) {
         return;
       }