Fastcat cannot start with the automatically generated fastcat_saved_positions.yaml when there are no actuators

[d-loret]

Fastcat has issues starting up an EtherCAT networks without actuators if a fastcat_saved_positions.yaml is generated automatically.
Without actuators in the topology and actuator_fault_on_missing_pos_file set to False, Fastcat can be brought up the first time. An empty fastcat_saved_positions.yaml is created at the end.

When Fastcat is attempted to be brought up a second time, the following error occurs:

08/04/22 16:38:04 (1659656284.394366) [DEBUG]                     manager.cc:837 | Opening Pos File: /cal//fastcat_saved_positions.yaml
08/04/22 16:38:04 (1659656284.394389) [ERROR]                 yaml_parser.cc:12  | Expecting YAML Node: actuators
terminate called after throwing an instance of 'std::invalid_argument'
  what():  Fastcat Manager failed to process bus configuration YAML file.

To start up Fastcat again requires to manually delete the created fastcat_saved_positions.yaml or adding an actuators: line to the file.

When there are no actuators in the topology, Fastcat should 1) appropriately handle an empty fastcat_saved_positions.yaml without faulting, or 2) add an actuators: line to fastcat_saved_positions.yaml when it creates the file automatically.

[d-loret]

Seems that we need more logic around here: https://github.com/nasa-jpl/fastcat/blob/master/src/manager.cc#L831-L834