Fix index for merged train cars

What does this MR do and why?

Resolves #465252 (closed)

This MR modifies MergeTrains::Car#index to return nil if the car is in an active state (not merging or merged).

Right now, Car#index successfully ticks down for active cars on the train, but all merged cars show an index of 0 because they can't go any lower. The car asks the train, of all active cars on the same Train, how many have a lower id? But it doesn't specifically check the status of the car asking in the first place. So once it reaches 0, it will just stay 0 forever.

I added a pretty simple check to the top of the index method. The scope that MergeTrains::Train#all_cars uses is active, built by the same logic that builds the active method we use here.

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