(maint) Return nil on sysctl command lookup fail

See also 7fc668519f8e.
Merge branch 'maint/stable/solaris-missing-sysctl-command' into stable

One would think I would just fix all of these issues in a single branch.
One would be more reasonable than I.
Merge branch 'stable' into facter-2
Merge branch 'facter-2'