Fix a crash when the `EnableDiskWatch` command line argument is enabled

[Basilisk3]

Description of the proposed changes

Validate that the unit has a Physics table before assigning the TransportSpeedReduction stat. This allows us to better test changes without reloading the game.

Testing done on the proposed changes

Tested by removing the Physics table of a unit while having the EnableDiskWatch command line argument enabled.

Checklist

• [x] Changes are documented in the changelog for the next game version