Don't coerce nil for collection attributes when required is false

[mherold]

Without this fix, collection attributes can never be set to nil when coercion is enabled (because the coercer coerces nil to an empty array).

This should fix #334.

I originally suggested a change in a more central location, see this comment, but looking at the specs it seemed more appropriate to make this change only for collection attributes.

[forest]

:+1: