kunit: update NULL vs IS_ERR() tests

The alloc_string_stream() functions were changed from returning NULL on
error to returning error pointers so these caller needs to be updated
as well.

Fixes: 78b1c658 ("kunit: string-stream: Simplify resource use")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>