Blender Git Loki

RNA: Make is clear that `Scene` parameter of `update` callback may be NULL.

There are cases where there is no way to ensure we do have/know about an
active scene. Further more, this should not be required to perform
'real' updates on data, only to perform additional special handling in
current scene (mostly related to editing tools, UI, etc.).

This pointer is actually almost never used in practice, and half of its current
usages are fairly close to abuse of the system (like calls to
`ED_gpencil_tag_scene_gpencil` or `BKE_rigidbody_cache_reset`).

This commit ensures that the few places using this 'active scene' pointer are
safely handling the `NULL` case, and clearly document the fact that a
NULL scene pointer is valid.