Build with single generation mode failed because
new_space_allocation_top() and new_space_allocation_limit() both return
nullptr now without a new space. Previously the DCHECK succeeded because
both methods would call the NewSpace methods with null as this pointer.

Bug: v8:11708
Change-Id: I74babded2c790642e74722ed53794aecebec4344
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2917604Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74794}