Fix Encoding regression

[tarekgh]

#110521

The change here is fixing a regression from the change #97950.
The regression occurs in the multi-byte encoding when handling a surrogate pair. It incorrectly throws an exception when the character buffer is null, which is not the expected behavior. The code should be resilient to a null buffer, avoiding exceptions and instead returning only the count of the decoded characters.

The change is not reverting #97950 as the rest of the changes are beneficial.

[tarekgh]

CC @manandre @ericstj

[tarekgh]

The fix of this issue needs to be backported to the 9.0 servicing release.

[ericstj]

Seems like System.Text.Tests.EncodingTest.TestDefaultEncodings is failing on many jobs. Could be related.

Error message
Assert.Throws() Failure: No exception was thrown
Expected: typeof(System.NotSupportedException)

Stack trace
   at System.Text.Tests.EncodingTest.TestDefaultEncodings() in /_/src/libraries/System.Text.Encoding.CodePages/tests/EncodingCodePages.cs:line 481
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args) in /_/src/mono/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.Mono.cs:line 22
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(O

runtime/src/libraries/System.Text.Encoding.CodePages/tests/EncodingCodePages.cs

Line 481 in 812d504

Assert.Throws<NotSupportedException>(() => Encoding.GetEncoding((int)mapping[0]));

[tarekgh]

Seems like System.Text.Tests.EncodingTest.TestDefaultEncodings is failing on many jobs. Could be related.

I'll take a look

[tarekgh]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/runtime/actions/runs/12754927051

[tarekgh]

/ba-g the failing build is timeout which happened in other PRs.