Fix ListBox.rows_max calculation for empty container

[penguinolog]

In case of empty container,
get_focus will return None, None

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 9775555283

Details

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 74.746%

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Totals
Change from base Build 9739331036:	0.005%
Covered Lines:	9413
Relevant Lines:	12741

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💛 - Coveralls