Stop CpuSet.NodeSetsAreNonEmpty assuming every NUMA node has CPUs

[graphicsMan]

Fixes #89.

The test asserts that every NUMA node reports at least one CPU:

for (int32_t i = 0; i < numNodes; ++i) {
  EXPECT_GT(CpuSet::node(i).count(), 0) << "NUMA node " << i << " has no CPUs";
}

That is not true in general. A NUMA node with no CPUs is legal, and at least two
common cases produce one: memory-only nodes (CXL-attached memory, persistent
memory) and nodes whose CPUs have all been taken offline. Both report an empty
cpulist.

On such a host the test fails with NUMA node 1 has no CPUs while CpuSet is
reporting the topology entirely correctly — the failure is in the test's
assumption, not the library.

This sums across nodes and asserts the total is positive, which is the property
actually worth holding: that node enumeration finds CPUs somewhere. The
stronger claim the test appears to be reaching for, that the node sets partition
all(), is already covered by AllSetCoversAllNodeSets.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.0%. Comparing base (d74898a) to head (f8a17b8).

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #86     +/-   ##
=======================================
+ Coverage   92.6%   93.0%   +0.3%     
=======================================
  Files         64      64             
  Lines       4990    4990             
  Branches     678     682      +4     
=======================================
+ Hits        4625    4643     +18     
+ Misses       365     347     -18     

see 10 files with indirect coverage changes

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