nodedevmdevctltest: Rename mdevctl-list-empty test case

nodedevmdevctltest: Rename mdevctl-list-empty test case

The mdevctl-list-empty test case is there to test whether an
empty JSON array "[]" is handled correctly by mdevctl handling
code. Well, mdevctl can output both, an empty JSON array or no
output at all.

Therefore, rename "mdevctl-list-empty" test case to
"mdevctl-list-empty-array" which is more descriptive and also
frees up slot for actual empty output (handled in next commits).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>