tests: lib: custom_lib: use YAML empty dictionary syntax

There is no need to use dummy properties when an empty dictionary
can be used.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

Author: gmarull

tests/lib/custom_lib/testcase.yaml

@@ -4,7 +4,6 @@ common:
     - custom_plank
     - qemu_cortex_m0
 tests:
-  lib.custom_lib:
-    build_only: false   # something benign to make a non-empty list
+  lib.custom_lib: {}
   lib.custom_lib.non_default:
     extra_args: CONFIG_CUSTOM_LIB_GET_VALUE_DEFAULT=6