fix: Debug toolbar logs controller

[mjomble]

Description
Fixed bug introduced in #9581 and described in this comment

• isEmpty() was checking the opposite
• The checks did not work if $this->data was null

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[mjomble]

I'll look into adding a unit test for the Logs controller

[mergeable]

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