kunit: Don't crash if no parameters are generated

It's possible that a parameterised test could end up with zero
parameters. At the moment, the test function will nevertheless be called
with NULL as the parameter. Instead, don't try to run the test code, and
just mark the test as SKIPped.

Reported-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>