Update SqlVector<T>.Null refs

[frankbuckley]

Updates refs for SqlVector<T>.Null to reflect it returns Nullable<SqlVector<T>>.

Should fix #3520

[Copilot]

Pull Request Overview

This PR updates the return type of the SqlVector<T>.Null property from SqlVector<T> to SqlVector<T>? (nullable) in both .NET Framework and .NET Core reference assemblies to correctly reflect that it returns a nullable SqlVector<T>.

• Updates the property signature to use nullable reference types syntax
• Maintains consistency between .NET Framework and .NET Core implementations

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Microsoft.Data.SqlClient/netfx/ref/Microsoft.Data.SqlClient.cs	Updates SqlVector<T>.Null property return type to nullable in .NET Framework reference assembly
src/Microsoft.Data.SqlClient/netcore/ref/Microsoft.Data.SqlClient.cs	Updates SqlVector<T>.Null property return type to nullable in .NET Core reference assembly

[benrr101]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[benrr101]

I think the major note is that this change is updating the ref definition to match the implementation definition.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.13%. Comparing base (b89ac49) to head (9061eb2).
⚠️ Report is 3 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (b89ac49) and HEAD (9061eb2). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (b89ac49)	HEAD (9061eb2)
addons	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3521      +/-   ##
==========================================
- Coverage   65.01%   59.13%   -5.89%     
==========================================
  Files         276      270       -6     
  Lines       62414    62098     -316     
==========================================
- Hits        40581    36722    -3859     
- Misses      21833    25376    +3543     

Flag	Coverage Δ
addons	?
netcore	63.54% <ø> (-5.58%)	⬇️
netfx	60.90% <ø> (-5.80%)	⬇️

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