Use an explicit comparison to None for the converter of a field

[filbranden]

Summary

Make attrs work with a converter instance that does not evaluate True. Other code spots already do an explicit comparison to None, so do that here as well. Failing to do so on a valid converter instance that is a valid callable but has a __bool__() that evaluates to False ends up trying to call _fmt_converter_call on that callable rather than on the Converter instance created from it.

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[filbranden]

@hynek Would you kindly take a look whenever possible? Thanks!