Merge pull request #1443 from OneSignal/fix/remove-keys-from-SharedPreferences

Remove a key from Shared Prefs if its value is `null`

Author: nan-li

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalPrefs.java

@@ -204,6 +204,8 @@ else if (value instanceof Long)
                             editor.putLong(key, (Long)value);
                         else if (value instanceof Set)
                             editor.putStringSet(key, (Set<String>)value);
+                        else if (value == null)
+                            editor.remove(key);
                     }
                     prefHash.clear();
                 }