Fix nullable annotations in ITypedList

[gpetrou]

According to:
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedlist.getitemproperties
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedlist.getlistname
the parameters can be null.
I bumped into this problem while annotating ListBindingHelper in WinForms repository.
https://github.com/dotnet/winforms/blob/bc73fc2790fdce8ac7399eddf76824161bdd35cc/src/System.Windows.Forms/src/System/Windows/Forms/ListBindingHelper.cs

[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]

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Issue Details

---

According to:
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedlist.getitemproperties
https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.itypedlist.getlistname
the parameters can be null.
I bumped into this problem while annotating ListBindingHelper in WinForms repository.
https://github.com/dotnet/winforms/blob/bc73fc2790fdce8ac7399eddf76824161bdd35cc/src/System.Windows.Forms/src/System/Windows/Forms/ListBindingHelper.cs

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

[stephentoub]

LGTM. Thanks.