units: Fix unit definition logic

[leroyvn]

Description

There was a bug where a unit definition that would not already exist would not be loaded. This PR fixes it.

Checklist

• The code follows the relevant coding guidelines
• The code generates no new warnings
• The code is appropriately documented
• The code is tested to prove its function
• The feature branch is rebased on the current state of the main branch
• I updated the change log if relevant
• I give permission that the Eradiate project may redistribute my contributions under the terms of its license