Avoid premature trimming in RegionIntersection

[SirYwell]

Overview

Fixes #2470

Description

RegionIntersection can delegate to Region#processSet of one of the regions only if only this region overlaps with the chunk to process.
Otherwise, we need to check if any region contains a block before we can trim it, i.e. delegate to the default implementation of processSet.

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[TheMeinerLP]

LGTM