Fix nullability on IDesignerSerializationProvider.GetSerializer

[elachlan]

Related to ongoing work to null annotate the winforms codebase.

• Refactor DesignerSerializationManager to replace HashTable and enable Nullability winforms#8353
• https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.design.serialization.idesignerserializationprovider.getserializer?view=net-7.0

The annotations for IDesignerSerializationProvider.GetSerializer() need to be updated to reflect nullability for params currentSerializer and objectType as these parameters should allow null.

dotnet/winforms#8353 (comment)

[ghost]

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Issue Details

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Related to ongoing work to null annotate the winforms codebase.

• Refactor DesignerSerializationManager to replace HashTable and enable Nullability winforms#8353
• https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.design.serialization.idesignerserializationprovider.getserializer?view=net-7.0

The annotations for IDesignerSerializationProvider.GetSerializer() need to be updated to reflect nullability for params currentSerializer and objectType as these parameters should allow null.

dotnet/winforms#8353 (comment)

Author:	elachlan
Assignees:	-
Labels:	area-System.ComponentModel, community-contribution
Milestone:	-

[elachlan]

CC: @lonitra @KlausLoeffelmann