fix #23885 - Parallel GC has intermittent long collection pauses …

[rainers]

…under single-CPU affinity on Windows and WSL2

don't set the evStackFilled condition if the stack is empty in case very few roots are found: In that case other threads spin without getting any scan range because the condition and the scanStack are out of sync.

Detected by looking at xperf traces. No idea how to setup a reliable test case.

[github-actions]

DMD perf check

Metric	Base	PR	Δ
compile Phobos codegen (instr)	1,429.0 M	1,431.0 M	+0.140%
dmd binary size (stripped)	8.14 MB	8.12 MB	-0.14%

All measurements

Metric	Base	PR	Δ
compile hello.d (instr)	211.6 M	211.6 M	+0.016%
compile hello.d -O -release (instr)	229.2 M	229.2 M	+0.020%
compile Phobos (instr)	4,903.0 M	4,905.1 M	+0.042%
compile Phobos codegen (instr)	1,429.0 M	1,431.0 M	+0.140%
compile vibe.d (instr)	14,495.5 M	14,497.9 M	+0.016%
dmd binary size (stripped)	8.14 MB	8.12 MB	-0.14%
hello binary size (stripped)	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	44.32 MB	44.18 MB	-0.32%
peak RSS (compile Phobos)	616.3 MB	616.1 MB	-0.04%
peak RSS (compile vibe.d)	1909 MB	1909 MB	0.00%
compile dmd itself (wall)	9.5 s	9.5 s	+0.26%
compile hello.d (wall)	50.6 ms	49.5 ms	-2.07%
compile Phobos (wall)	1,143 ms	1,154 ms	+0.91%

_{23c062d vs merge-base ace0c09 · about these metrics}