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

**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