Update SqlVector<T>.Null refs (#3521)

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

Author: frankbuckley

src/Microsoft.Data.SqlClient/netcore/ref/Microsoft.Data.SqlClient.cs

@@ -131,7 +131,7 @@ public SqlVector(System.ReadOnlyMemory<T> memory) { }
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/IsNull/*' />
         public bool IsNull => throw null;
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/Null/*' />
-        public static SqlVector<T> Null => throw null;
+        public static SqlVector<T>? Null => throw null;
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/Length/*' />
         public int Length { get { throw null; } }
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/Memory/*' />

src/Microsoft.Data.SqlClient/netfx/ref/Microsoft.Data.SqlClient.cs

@@ -2425,7 +2425,7 @@ public SqlVector(System.ReadOnlyMemory<T> memory) { }
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/IsNull/*' />
         public bool IsNull => throw null;
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/Null/*' />
-        public static SqlVector<T> Null => throw null;
+        public static SqlVector<T>? Null => throw null;
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/Length/*' />
         public int Length { get { throw null; } }
         /// <include file='../../../../doc/snippets/Microsoft.Data.SqlTypes/SqlVector.xml' path='docs/members[@name="SqlVector"]/Memory/*' />