[release/8.0.1xx-xcode15.4] [Foundation] Allow setting NSUrlSessionHa…

…ndler.Proxy to null (#21147)

Before PR:

NSUrlSessionHandler.Proxy property throws NSE when client code set any value (including `null`):

![image](https://github.com/user-attachments/assets/bb273910-3519-490f-b5fd-1999327012b3)

After PR

NSUrlSessionHandler.Proxy property throws NSE only when the client code
tries to set a value different from `null` (Custom proxy is not
supported right now in iOS implementation so setting a value other than
null must throws NSE):

![image](https://github.com/user-attachments/assets/4a564785-3e23-469c-ac26-460d028cf770)

Fixes #21132.

PR originally submitted by @adospace in #21137.

Co-authored-by: adospace <adospace@msn.com>

Backport of #21140

src/Foundation/NSUrlSessionHandler.cs

@@ -651,7 +651,10 @@ public bool PreAuthenticate {
  [EditorBrowsable (EditorBrowsableState.Never)]
  public IWebProxy? Proxy {
  get => null;
- set => throw new PlatformNotSupportedException ();
+ set {
+ if (value is not null)
+ throw new PlatformNotSupportedException ();
+ } 
  }
 
  // There doesn't seem to be a trivial way to specify the protocols to accept (or not)