Nullness improvements for Combine, Min, Max

[kennknowles]

This is a fairly small one, re-adding nullness checking to Combine and a couple specific combiners.

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[kennknowles]

Hmm, actually I think I lost the thread on this one - I'm pretty sure identity() should not be @Nullable T but rather the accumulator type has to be nullable if the combiner doesn't have a natural identity.

[kennknowles]

Ah, it turns out this is very hard to change - the mistake dates back to the beginning of Combine where we deliberately made all identities nullable and there is no way around it. So, the current PR isn't adding any nullability.

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[ahmedabu98]

LGTM

[kennknowles]

Ran internal test suite as well.