Allow Namespace pydantic model to have a null metadata_ entry

[soapy1]

Fixes #1062

Description

This PR updates the pydantic model for Namespace so that the metadata_ field can be null. This field is nullable, and the default value is an empty dict:

class Namespace(Base):
    """Namespace for resources"""

    . . .

    metadata_: Mapped[Optional[dict]] = mapped_column(JSON, default=dict, nullable=True)

    . . .

In order for pydantic models to allow values to be None, they must set them as Optional. If not, an error will be thrown.

In older deployments on conda-store it is possible for this field to actually be NULL (see the explanation #1062 (comment)), resulting in an error.

Additional notes

While sorting this out I also checked for other orm fields that were explicitly set to nullable=True and ensured that their pydantic counterparts are set to Optional.

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

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[peytondmurray]

I added a comment that provides context here, but otherwise looks good. 🚢