[P2] Fix evolutions unable to strictly apply to base Pokemon forms

[innerthunder]

What are the changes the user will see?

G-Max Pikachu, Eevee, Meowth, and Duraludon should no longer be incorrectly prompted or allowed to evolve.

Why am I making these changes?

Reported on the Discord

What are the changes from a developer perspective?

field/pokemon: Pokemon#getEvolution now uses isNullOrUndefined(evo.preFormKey) instead of !evo.preFormKey to check if an evolution isn't tied to a specific form. Using !evo.preFormKey caused issues because the keys for base forms are "", which is also interpreted as falsy.

Screenshots/Videos

How to test the changes?

Easiest way using overrides:

1. Give yourself a Level 28 G-Max Meowth.
2. Clear a wave. Then, from the rewards screen, give your G-Max Meowth a Rare Candy.
3. The Meowth should not be prompted to evolve into a Persian.

Checklist

• I'm using beta as my base branch
• There is no overlap with another PR?
• The PR is self-contained and cannot be split into smaller PRs?
• Have I provided a clear explanation of the changes?
• Have I considered writing automated tests for the issue?
• If I have text, did I make it translatable and add a key in the English locale file(s)?
• Have I tested the changes (manually)?
  • Are all unit tests still passing? (npm run test)
• [?] Are the changes visual?
  • [?] Have I provided screenshots/videos of the changes?