Allow Spacecraft Subsystems to be Empty

[Deantwo]

With the new designer, new manufacturing processes and refitting. It would be nice if spacecraft subsystems could be set to "empty".

For example designing a spacecraft blueprint with a shield console, but leave it with no module type selected. Then we could refit swap the empty module with a shield module once we have acquired needed materials.

With the ideas Haxus mentioned wanting to implement allowing us to change module types when starting manufacturing of a spacecraft, we could even set subsystems we haven't researched yet to empty.
Volume used on those subsystems could just be tossed into framework, or just remain unused and useless until a module is installed.

[Deantwo]

This would also remove the need for having the choice between "Spaceship" and "Starship", since you could just choose to not add a FTL drive module instead.