selftests: kselftest_deps: fix l5_test() empty variable

In the function l5_test(), variable $tests is empty when there is no .mk
file in the subsystem to be tested. It causes the following grep operation
get stuck.

This fix check the variable $tests, return when it is empty.

Signed-off-by: Lu Dai <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/kselftest_deps.sh

···
 l5_test()
 {
 	tests=$(find $(dirname "$test") -type f -name "*.mk")
+	[[ -z "${tests// }" ]] && return
 	test_libs=$(grep "^IOURING_EXTRA_LIBS +\?=" $tests | \
 			cut -d "=" -f 2)