fix: prevent reaction to undefined zoom

[silvester-pari]

Implemented changes

This PR introduces a small check to the zoom setter if the value is actually defined, as setting undefined resulted in a blank map.

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• I have performed a self-review of my code
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• The PR title follows conventional commit style and reflects the type of change (major/minor/patch, breaking)
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[RobertOrthofer]

looks good to me, can't think of any real side effect right now...