[Bug]: Refit in stations can fail if leading articulated part has 0 capacity

[JGRennison]

Version of OpenTTD

master

Expected result

Refit in stations works as expected

Actual result

Refitting in stations doesn't always work as expected because the cargo type of 0 capacity parts is not changed during refitting, so the test in HandleStationRefit can cause refitting to not be attempted because the 0 capacity leading part already matches the target cargo.

Steps to reproduce

See NewGRFs such as eGRVTS r188.

[xplusc]

Which vehicles from eGRVTS and cargos exhibit this behavior? I tested every refittable tram and multiple refittable road vehicles from eGRVTS r202 and they all worked correctly. Is this specific to eGRVTS r188?