[PS-1368] Make Entities `Nullable`

[justindbaur]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Add nullable annotations on all of our types that correspond to tables in SQL.

Code changes

• Most Added #nullable to all entities files and changed their annotations to abide by their SQL constraints.
• src/Api/Controllers/OrganizationExportController.cs: Fixed a dotnet format complaint

Before you submit

- [x] I have checked for formatting errors (`dotnet format --verify-no-changes`) (required)
- [ ] If making database changes - I have also updated Entity Framework queries and/or migrations
- [ ] I have added **unit tests** where it makes sense to do so (encouraged but not required)
- [ ] This change requires a **documentation update** (notify the documentation team)
- [ ] This change has particular **deployment requirements** (notify the DevOps team)

[justindbaur]

@MGibson1 Probably just a couple BEEEP days if I can get the reviews, my plan is Entities (this) -> Repositories -> Services -> Rest of src/* and then we can discuss if we want it on in tests or not.