kunit: string-stream: Don't create a fragment for empty strings

If the result of the formatted string is an empty string just return
instead of creating an empty fragment.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>