fix(asciidoc): set default size when missing in image directive (#1769)

The AsciiDoc backend should not create an ImageRef with Size equal to None, instead use default size values.
Refactor static methods as such and add the staticmethod decorator.
Extend the regression test for this fix.

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>