GRIM/EMI: Handle special case in Actor::walkForward()

The walk chore of the monkey bot is simulating
a robotor-like walking style by setting _walkRate to
0.0f repeatedly. This prohibits walking forward
which results in stopping the walk chore eventually.

To avoid the walk chore in this case to stop, assume that
the actor has walked when calling Actor::walkForward() with
a _walkRate of 0.0f.

This fixes #1112.