Replace null log file extension check

[MGatner]

Description
The recent Logger change to default to .log added a null check for config->fileExtension, but since this is defined in app/Config/Logger.php it won't normally be null, just empty.

This PR checks for an empty value (could also use ?: but this covers cases where it isn't set at all).

Checklist:

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