[Moco] Fix intermediate callback function for `MocoCasADiSolver`

[nickbianco]

Currently, setting the 'output_interval' property for MocoCasADiSolver causes problems to immediately fail.

Reported by Reed Gurchiek.

[nickbianco]

See #3000.

[nickbianco]

Removing the integer format specification :06i in this line prevents crashing, but the intermediate trajectories are still all zeros.