Caused by rBcf2baa585cc8: Fix T81707: Spline IK Joints "Floating" above curve.

For Curve Guide force fields to work, the Path Animation option has to
be enabled. With it disabled, we are lacking the necessary
anim_path_accum_length data initialized [done by
BKE_anim_path_calc_data] which BKE_where_on_path relies on since
above commit.

Now just check for this before using it - and return early otherwise.
Prior to said commit, BKE_where_on_path would equally return early
with a similar message, so that is expected behavior here.