Properties such as Mesh.shape_keys can be None, so should be Optional

[Mysteryem]

Some properties can be None and this should be indicated by declaring them with typing.Optional.

Properties do have an is_never_none attribute, but it's mostly used for properties that have an equivalent 'falsey' value, such as string properties that will be '' when there is no value set. Additionally, is_never_none can't be trusted, since bpy.types.Object.image_user has is_never_none, yet can actually be None.

I don't know if there's actually a way to tell if a property is optional. The is_required attribute sounds promising, but appears to only be used for function parameters.