Fix nullable annotations in ITypeDescriptorContext

[gpetrou]

According to:
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedescriptorcontext.container?view=net-7.0
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedescriptorcontext.instance?view=net-7.0
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedescriptorcontext.propertydescriptor?view=net-7.0
these can be null.
I bumped into this problem while annotating GridEntry in WinForms repository.
https://github.com/dotnet/winforms/blob/ecc6966e1eb76d7c9921638137e058f2243d7dd9/src/System.Windows.Forms/src/System/Windows/Forms/PropertyGridInternal/GridEntry.cs#L170

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-componentmodel
See info in area-owners.md if you want to be subscribed.

Issue Details

---

According to:
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedescriptorcontext.container?view=net-7.0
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedescriptorcontext.instance?view=net-7.0
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedescriptorcontext.propertydescriptor?view=net-7.0
these can be null.
I bumped into this problem while annotating GridEntry in WinForms repository.
https://github.com/dotnet/winforms/blob/ecc6966e1eb76d7c9921638137e058f2243d7dd9/src/System.Windows.Forms/src/System/Windows/Forms/PropertyGridInternal/GridEntry.cs#L170

Author:	gpetrou
Assignees:	-
Labels:	area-System.ComponentModel, new-api-needs-documentation, community-contribution
Milestone:	-

[stephentoub]

Thanks!

[stephentoub]

@jeffhandley, I assume we'll do a single breaking change roll-up page again for all nullable annotation differences that would lead to new warnings?

[jeffhandley]

@stephentoub Thanks for checking on this. We will not do a single roll-up doc issue for nullable annotation changes, as that didn't work out great for doc creation tracking. The changes might all get rolled up into a single doc, but we need to file individual issues.

I created dotnet/docs#33225 for this one.