Return no value when global memory scheduling is off. Previously
returned new_space->Capacity()+1 since this value was used in the
callers to compare the result to.

Bug: v8:10315
Change-Id: I7c7289ee9319fe9c791c3cb9b9b46cca54815904
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235704
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68251}