kunit: Don't fail test suites if one of them is empty

Empty test suite is okay test suite.

Don't fail the rest of the test suites if one of them is empty.

Fixes: 6ebf5866 ("kunit: tool: add Python wrappers for running KUnit tests")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>