Fix error when loading a model that has net_arch manually set to None

[jak3122]

Description

Changed the conditional when loading the policy_kwargs to handle the case where the saved net_arch is None, simplified the code a bit, and added a test.

Motivation and Context

I ran into this issue when using a wrapper script that potentially passes None to the net_arch parameter, and then when I go to load the saved model, there is an error.

closes #1928

• [x] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)

Checklist

• [x] I've read the CONTRIBUTION guide (required)
• [x] I have updated the changelog accordingly (required).
• [ ] My change requires a change to the documentation.
• [x] I have updated the tests accordingly (required for a bug fix or a new feature).
• [ ] I have updated the documentation accordingly.
• [ ] I have opened an associated PR on the SB3-Contrib repository (if necessary)
• [ ] I have opened an associated PR on the RL-Zoo3 repository (if necessary)
• [x] I have reformatted the code using make format (required)
• [x] I have checked the codestyle using make check-codestyle and make lint (required)
• [x] I have ensured make pytest and make type both pass. (required)
• [x] I have checked that the documentation builds using make doc (required)

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[qgallouedec]

It's weird because I see you've pushed a fix to my comment on your branch, but I can't see it on the PR page. First time I've seen that.

[jak3122]

For around a day the PR page said it was processing something. Currently it says "1 workflow awaiting approval". I did forget to use the -u flag when pushing the branch originally, and only used it for the new commit, so I wonder if that broke something. I'll try pushing a dummy commit.

[jak3122]

Alright now I see the fix commit on the PR page, hope that fixed it!

[qgallouedec]

LGTM, thanks @jak3122! You can add you id to the name of all the contributors (see bottom of changelog)