Treat memory-only NUMA nodes as valid topology

[joejulian]

Memory-only NUMA nodes are valid Linux topology, but GetNodesInfo currently emits a warning whenever a node has no CPU directories. On systems that intentionally expose memory-only nodes this warning repeats on every topology refresh.

Treat an empty CPU set as a valid memory-only node and move that diagnostic to V(4). Return actual GetCPUsPaths failures instead of conflating them with the valid empty result.

This follows up #3828.

Tests added for:

• retaining a memory-only NUMA node and its memory data without a warning-level failure path
• propagating a real CPU-path discovery error

Validation:

• go test ./utils/sysinfo (from lib/)
• go test -race ./utils/sysinfo (from lib/)
• go vet ./... (root and lib/)
• gofmt -l .

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[joejulian]

@dims since you touched this file last, would you please take a look?