BUG: `Flight` without `RailButtons` crash on `get_instance_attributes`

[phmbressan]

Describe the bug

A Flight that has a Rocket without rail buttons will crash when calling get_instance_attributes, since the normal forces were not computed.

To Reproduce

1. Create a `Rocket` without rail buttons;
2. Make a `Flight` with this rocket;
3. Import and run `get_instance_attributes` over it.

For instance, removing the rail buttons of the getting_started example.

Expected behavior

The method get_instance_attributes should show the attributes correctly, without throwing. A default or null value could be used for the rail button forces.

Additional context

There likely should be more tests and validation around get_instance_attributes to make sure degenerate conditions don't arise for certain inputs (e.g. edge cases).