Can't load population into cargo when there is already population of the same race (even zero) in cargo

[ekolis]

Describe the bug
If there is population of a particular race in a ship's cargo, even with a quantity of zero, additional population of that race cannot be loaded into that ship's cargo.

Steps to reproduce

1. Load population into a ship's cargo
2. Drop the population somewhere
3. Try to load more population into the ship's cargo

Expected behavior
The population is loaded into cargo.

Actual behavior
The population is not loaded into cargo, and the ship shows having a population quantity of zero in cargo.

Screenshots
N/A

Related issues
N/A

Testing plan
Write a unit test for cargo transfer after confirming that's where the bug is

Additional context
N/A

[ekolis]

Seems to be a problem with the way load cargo orders are created. It's creating an order to load all population of a null race, which doesn't mean anything, instead of to load all population of a specific race or to load all population regardless of race. Not sure why colony ships still load population, they must be using a different load cargo order.

[ekolis]

Looks like Bearclaw was trying to load all population of a specific race. The race ID is saved correctly into the plr file, but for some reason when processing the turn, it's null. The race population amounts are stored in a GameReferenceKeyedDictionary in the TransferCargoOrder; maybe something is wrong with the deserialization of that class?

[ekolis]

Still seems to be happening. Might have something to do with population being stored in cargo as a SafeDictionary instead of a ReferenceKeyedDictionary; that could be related to duplicate race entries appearing in cargo or population (#203), if the race object is being duplicated.

[ekolis]

The same issue is happening when unloading population onto a colony with zero population. Something about there being zero population in cargo or on a colony is preventing more population from being added.

[ekolis]

Cargo should be able to store population without using a ReferenceKeyedDictionary though, because it's not actually sent from the client to the server. CargoDelta is, and it does use a ReferenceKeyedDictionary.