Fix logic for inverting a BooleanIndex when documents are unindexed

[davisagli]

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Closes #20

[ale-rt]

Thanks! This also makes the check consistent with https://github.com/zopefoundation/Products.ZCatalog/pull/160/files#diff-4b2953531845fded8a643f461155adcf0369f014cdaaa40ee207dd3e1fe2b339L127-L131

[davisagli]

@icemac Could you make a release please?

I could also take care of this if you would like to give my PyPI access to do so (davisagli). /cc @tseaver

[tseaver]

@davisagli "Poof! You're a mainatiner!"

[davisagli]

thanks @tseaver

[icemac]

Just released as https://pypi.org/project/Products.ZCatalog/7.2.1/

[davisagli]

@icemac Thank you!