fix: config max zoom 0

[iwpnd]

as per #870, setting max_zoom to 0 currently causes the layer to bleed through over the entire zoom range. Either we fix filtering as per #871 or make sure max_zoom is never 0 as I do in this PR.

[coveralls]

Pull Request Test Coverage Report for Build 41ebf1ba3-PR-874

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 45.309%

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Totals
Change from base Build a78880d9a:	0.04%
Covered Lines:	5607
Relevant Lines:	12375

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💛 - Coveralls