bindings: python: fix Line.request() crashing

bindings: python: fix Line.request() crashing

On an attempt to call the 'request' method of a Line object, the program
crashes with this exception:

> SystemError: ../Objects/dictobject.c:2606: bad argument to internal function
>
> The above exception was the direct cause of the following exception:
>
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> SystemError: <class 'gpiod.LineBulk'> returned a result with an error set

The problem is that keyword args are NULL (rather than an empty dict) if
they are not present. However, PyDict_Size sets an exception if it gets
NULL.

Fixes: 02a3d0a2ab5e ("bindings: python: fix segfault when calling Line.request()")
Signed-off-by: Jiri Benc <jbenc@upir.cz>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>