Account for existing but empty cpus file

If a CPU file exists but is empty, previously we reported this file with a CPU count of 1, which resulted in a single-threaded run.

Author: angelapwen

CHANGELOG.md

@@ -6,7 +6,7 @@ Note that the only difference between `v2` and `v3` of the CodeQL Action is the
 
 ## [UNRELEASED]
 
-No user facing changes.
+- Fix an issue where an existing, but empty, `/sys/fs/cgroup/cpuset.cpus` file always resulted in a single-threaded run. [#2151](https://github.com/github/codeql-action/pull/2151)
 
 ## 3.24.3 - 15 Feb 2024