Fix T41766: FBX (binary) exporter does not export actions with one frame animations.

When we are exporting actions or strips from NLA, and an evaluated anim leads to no anim
at all (like single-keyed actions or pure static ones), key all start and end frames.

Note this might be overkill (due to baked anim, we will key *all* transform channels),
but probably better than skipping completely those actions!