[stable] Fix Issue 22745 - std.parallelism.parallel fails to process empty range on a single CPU

[kinke]

Targeting stable because it's a pretty embarrassing bug and was just encountered at work, in a VM with a single CPU core.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22745	normal	std.parallelism.parallel fails to process empty range on a single CPU

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + phobos#8633"

[kinke]

@CyberShadow: Looks like the DAutoTest job is broken against stable.

git clone -b v2.101.0 --depth=1 https://github.com/dlang/druntime .generated/druntime-2.101.0
Cloning into '.generated/druntime-2.101.0'...
warning: Could not find remote branch v2.101.0 to clone.
fatal: Remote branch v2.101.0 not found in upstream origin
posix.mak:580: recipe for target '.generated/druntime-2.101.0' failed
make: *** [.generated/druntime-2.101.0] Error 128

[CyberShadow]

DAutoTest just runs the makefiles, it contains almost no logic like that on its own.

I think dlang/dlang.org#3455 should fix that once it's merged back into stable. CC @ibuclaw

[Geod24]

Nice, thanks!