code: public TableRef notNull(String attributeName){
  		filters.add(new Filter(StorageFilter.NOTNULL, attributeName, null, null));
  		return this;
  	}
docs: Applies a filter to the table reference. When fetched, it will return the non null values.
  
  <pre>
  StorageRef storage = new StorageRef("your_app_key", "your_token");
  
  TableRef tableRef = storage.table("your_table");
  
  // Retrieve all items where their "itemProperty" value is not null
  tableRef.notNull("itemProperty").getItems(new OnItemSnapshot() {
  &#064;Override
  public void run(ItemSnapshot itemSnapshot) {
  if (itemSnapshot != null) {
  Log.d("TableRef", "Item retrieved: " + itemSnapshot.val());
  }
  }
  }, new OnError() {
  &#064;Override
  public void run(Integer code, String errorMessage) {
  Log.e("TableRef", "Error retrieving items: " + errorMessage);
  }
  });
  
  </pre>
  
  @param attributeName
  The name of the property to filter.
  @return Current table reference