[5.4] Fix return type for App getDocument, getLanguage methods

[Fedik]

Pull Request for Issue # .

Summary of Changes

Fix return type for App getDocument() and getLanguage() methods.

Note: These objects are not initialized in the Application constructor but at different stage of Application execution and so cannot be always available.

I not sure where this PR should go, made to 5.4 for now.

Testing Instructions

Code review

Link to documentations

Please select:

• Documentation link for docs.joomla.org:
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org:
• No documentation changes for manual.joomla.org needed

[richard67]

I have tested this item ✅ successfully on ff85927

Code review.

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