Blender Git Loki

Fix T89805: NLA crash without active track

Was reported for a file which does not have an active track set in
AnimData even though it was in strip twek mode (but this was accessed in
is_nlatrack_evaluatable()).

Root cause for this is not totally clear, but I assume the situation is
described as part T87681 (and is fixed in D11052).

This patch here just prevents the crash for files that are already in the
borked state.

Reviewers: sybren

Maniphest Tasks: T89805

Differential Revision: https://developer.blender.org/D12085