Update model fields for consistent usage of Optional and default

[zanieb]

Makes two readability and consistency changes to our models:

• Use Optional for types when field defaults are None
  • Avoids relying on implicit optional behavior
  • Avoids confusion over unvalidated default values of None
• Always use default= explicitly
  • Static typing for default values will not display correctly otherwise, see
    BaseSettings - pylance complains for not setting default values to fields pydantic/pydantic#3753
    https://github.com/microsoft/pyright/blob/main/specs/dataclass_transforms.md#field-descriptors
  • Explicit is better than implicit

There should be no change in behavior here. This was performed with some crazy regular expressions then manual edits for edge cases. Please give it a second look.

The lack of explicit Optional types lead to issues in #6813 — this change should unblock that.

Example

The following:

class Model(BaseModel):
   foo: str = Field(None)

has changed to:

class Model(BaseModel):
   foo: Optional[str] = Field(default=None)

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• This pull request includes tests or only affects documentation.
• This pull request includes a fix, feature, enhancement, docs, collections, or maintenance label categorizing the change.

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[bunchesofdonald]

Looks good! I was a bit cross-eyed by the end, but all I found a couple places where Optional didn't need to be imported.

[zanieb]

Thanks! Yeah I got to a point where I'd made all these changes and then everything was failing because Optional wasn't available so I added it to literally every import of typing then relied on autoflake8 to drop it. A few places that doesn't go though :)

[chrisguidry]

This makes a lot of sense to me, and the only change we might need to check in on with Prefect Cloud is the ORMBaseModel. I'm not expecting any trouble, what you've done here is totally compatible and just making the implicit explicit (thank you!)