fix: treat 0 as false value

[mrexox]

Closes https://github.com/coverallsapp/orb/pull/28

:zap: Summary

Treat 0 as falsey value of COVERALLS_PARALLEL

[coveralls]

Pull Request Test Coverage Report for Build 4636394059

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.815%

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Totals
Change from base Build 4627625178:	0.0%
Covered Lines:	679
Relevant Lines:	756

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💛 - Coveralls

[zackse]

Thanks for the quick turnaround on this, much appreciated!