[fix] unable to save assets when updateNulls = true

#### Steps to reproduce the issue

We need a table with `$updateNulls` set to true on `store()` method. Let's for example use content articles. Edit file:

`libraries/legacy/table/content.php`

and modify the `store` method declaration from:

```php
public function store($updateNulls = false)
```

to: 

```php
public function store($updateNulls = true)
```

That will try to update null fields on save. Someone decided that the asset table should inherit it.

Now in backend open any article and click `Save`.
 
#### Expected result

Article & the corresponding asset are saved correctly.

#### Actual result

Page is reloaded without saving message. What happened internally is that the article was saved but the asset was not.

#### Pull request testing

After applying the patch on this PR try to save the article again. All should be saved correctly.

#### Backward compatibility

There is no B/C issue as it ensures that any table extending `JTableAsset` with an `alias` field still works.

#### System information (as much as possible)

Joomla: Latest staging 3.3.x version.

#### Additional comments

Remember to edit back `libraries/legacy/table/content.php` !! :dash: