processing AI trains raises redundant exception

[cjakeman]

The AITrain.AIUpdate() method sends signals to turn wipers on/off for AI trains depending on the rainfall.

If the first car in the train is not an MSTSLocomotive, then a NullReferenceException occurs which is then discarded with a:

catch {}

When developing, this fills the log with exception messages.
In production, it wastes CPU cycles.

I propose to replace the try {} catch {} with a check for != null