Posible Bug: Some vehicles without cab show up on an empty convoy

[Mariculous]

Some vehicles that do not have a front cab show up when assembling a new, yet empy convoy, whilst others don't.

Preciesely, convoys with prev "any" (besides a list of vehicles) will show up, whilst such with no constraints at all won't.
For sure you cannot send such vehicles out of the depot. They will report "Cannot start: no cab at the front of the convoy"

For consistency and clarity reasons, I'd suggest to hide all vehicles that do not have a front cab, when assembling a new convoy.

Both vehicles are defined bidirectional=1; has_front_cab=0.
The first got a prev "none" besides a list of vehicles defined, whilst the latter doesn't have any prev constraints defined.
Note that in the second image I had checked the "show all" checkbox to make it show up, whilst in the first case, It showed up anyways.