[Bug Fix] Serialization of vectorActionSpace

[vincentpierre]

Proposed change(s)

Fixing bug here : https://unity.slack.com/archives/C01H5LE96RE/p1617640959055900
The issue is that internal fields are not serialized by default like public fields and that the default DiscreteBranches is no longer null but Array.Empty.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[chriselion]

The fix itself looks fine. But considering we already broke this twice, we need a test that loads an old asset and makes sure the actions are preserved.