yaml: add back support for empty LED groups

The old script had support for empty LED groups. I tried removing the
empty "bmc_booted" LED group, but that caused systemd to never "finish"
booting (systemctl is-system-running returned "starting" because it was
blocked waiting for the "bmc_booted" LED service to start).

This adds back support for empty LED groups.

Tested:
Confirmed that with an empty "bmc_booted" LED group, the firmware will
build successfully and systemd doesn't get blocked waiting for the
"bmc_booted" LED service.

Change-Id: I11d7c50696cd50d989a4eaef28f8e5c43473ce6e
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/test/meson.build b/test/meson.build
index 4aa6632..b7f0b47 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -20,6 +20,7 @@
 endif
 
 test_yamls = [
+  'test-empty-group',
   'test-group-priority',
   'test-led-priority'
 ]
@@ -51,6 +52,7 @@
   'utest-group-priority.cpp',
   'utest-led-yaml-group-priority.cpp',
   'utest-led-yaml-led-priority.cpp',
+  'utest-led-yaml-empty-group.cpp',
   'utest-config-validator.cpp'
 ]
 if get_option('persistent-led-asserted').allowed()