Previously copying grease pencil strokes implicitly requires materials
to be available in `grease_pencil_copy_strokes_exec` which isn't
necessarily the case sometimes. Now just set material session uid to 0
when material is not found. When pasting, non-existent materials will
be created as a fallback already.
Additionally, this PR corrects material
access by using `BKE_object_material_get` instead of manually getting
the material from grease pencil material array, making it consistent
with the rest of the logic in blender.
Pull Request: https://projects.blender.org/blender/blender/pulls/142702
d6cc9a0793