Atomic reactor enable/disable + spam fix

[Josephos]

Changes to be committed:
modified: src/vehicle.cpp

Summary

SUMMARY: Bugfixes "Fix for issue #26617 "

Purpose of change

Fixes #26617 mini-reactor is always on regardless of setting, reactor with no fuel spams shutdown messages.

Describe the solution

Added some code to check whether or not the reactor is actually on. If there are no online reactors, there is nothing to turn off, so we can skip disabling them and spamming in the process.

Describe alternatives you've considered

I've considered leaving this alone and letting someone who knows what the hell is he doing to fix it instead.

[AMurkin]

You don't need count, only on/off state.

    bool reactor_online = false;
    for( auto &elem : reactors ) {
        reactor_online |= is_part_on( elem );
    }