Allow for slight movement when no engine equipped

[BenjaminAmos]

Currently when you eject/unequip all of your ship's engines, you are unable to move. Exiting the game and then continuing that save will restore your ship's default engine though. This is unintended behaviour but useful.

As @Cervator noted in #704 (review), it would be useful to allow for a tiny amount of thrust even with no engine equipped. This can be explained in the form of "emergency thrusters" or similar, so that the player is not completely helpless if they accidentally lose their only engine.