Fix session bug when the request is ajax call

[michalsn]

Description
When the session is initialized during ajax request and we use get(null) or get(key) with an empty $_SESSION variable, we always get the empty array as a result.

This pull request fixes it, so now we receive accordingly:

• empty array for get(null) call
• null for get(key) call

This is a very rare use case scenario.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide