Suggestion: "Any Cargo" interrupt wildcard reads filtered slots in empty cargo wagons

[deemacgee]

TL;DR

Allow train interrupt "any cargo" wildcard to read filtered slots in empty train wagons.

What?

Expand train interrupts to read wagon slot filters in empty wagons if no other cargo is present.

Why?

Allows an interrupt-driven train to specialise in a single cargo type without creating multiple parallel train groups.
Allows the "any cargo" wildcard to generate pickup request interrupts for empty trains, which it currently does not.

Thanks for reading!