Undefined titles on alert messages

This solves an issue with installation showing "undefined" alert messages reported in https://github.com/joomla/joomla-cms/pull/5331. Issue was introduced by me in https://github.com/joomla/joomla-cms/pull/5257

## Description

Basically the installation process is not adding alert message type translations properly (Before using any message type you have to add the JS translation with `JText::script('MESSAGE');` , `JText::script('ERROR');`, etc.). If not the message is not really translated. This is discovered after removing the jQuery dependency in https://github.com/joomla/joomla-cms/pull/5257 

Basically what I'm doing now is testing that the translation of the message type has been defined and if not we skip adding the alert heading. This preserves exactly the previous behavior. Of course it would be better to add that message types translations to the installer but I think this fix makes sense anyway if third part extension developers forget to add the translation for example.

### Backward compatibility

This PR has no B/C issues. In fact it fixes one.

## Test

On the first Joomla installation screen of Joomla do not fill any fields and press the `Next` button. You should see the `undefined` alert message titles. After applyting the patch no titles should be visible.