[Bug] Disabling sustainer on single stage rocket has no effect

[JoePfeiffer]

Describe the bug

When we disable all of the stages in a rocket and simulate it, the simulation should raise an exception. This works correctly for rockets with more than a single stage; a single stage rocket simulates successfully.

To Reproduce

1. Open "Simple Model Rocket" example
2. Disable sustainer. Rocket disappears from design window
3. Run first simulation. Simulation runs successfullly

Compare with the result of

1. Open "Two Stage Rocket" example
2. Disable both stages. Rocket disappears from design window
3. Run first simulation. Simulation throws an exception.

Screenshots / .ork file

Screenshot(s):

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.ork file:

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OpenRocket version

23.09, unstable. Haven't checked 22.02.

What platform are you running on?

Linux

Additional context

No response

[JoePfeiffer]

I'm not entirely sure it's doing the right thing on multistage, either after a little more looking...