update flexible_legal_basis_for_profiling boolean field to be optional

[adamsachs]

Closes n/a

Description Of Changes

as @NevilleS pointed out in the original PR just merged that added this field, we should have made the flexible_legal_basis_for_profiling field an Optional[bool] field on our PrivacyDeclaration model.

I'd thought this was a nitpick that could be ignored, but on further thought, i think this could have a legitimate functional impact! basically, for non-GVL based privacy declarations, we don't really want to specify whether or not they have a flexible legal basis. for GVL-based privacy declarations, we want to specify whether the legal basis is flexible or not.

i've tested this change with fides and the API/db layer, and things seem to work as expected. if the API request doesn't specify the field, it's left null in the db. if the API request does specify either true or false, the corresponding value gets persisted to the db 👍

Code Changes

• make the PrivacyDeclaration.flexible_legal_basis_for_processing model field an Optional[bool] with no default, i.e. defaults to None/null

Steps to Confirm

• tested fides API/db integration by using the fideslang alpha package based on this branch

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

[pattisdr]

Makes sense! Nice to catch this now