Allow null SdkResult from SdkResolver.Resolve (#13197)

The doc comment explicitly mentions returning null and the SDK can do
so, which caused a break in dotnet/dotnet#4579:


src\sdk\src\Resolvers\Microsoft.NET.Sdk.WorkloadMSBuildSdkResolver\WorkloadSdkResolver.cs(35,36):
error CS8764: Nullability of return type doesn't match overridden member
(possibly because of nullability attributes).

Follow up to #13157 / 0172c58.

Author: rainersigwald

src/Framework/Sdk/SdkResolver.cs

@@ -31,10 +31,9 @@ public abstract class SdkResolver
         /// applicable for a particular <see cref="SdkReference"/>.
         /// </returns>
         /// <remarks>
-        ///  Note: You must use <see cref="SdkResultFactory"/> to return a result.
-        ///  </remarks>
-        ///
-        public abstract SdkResult Resolve(SdkReference sdkReference,
+        /// Note: You must use <see cref="SdkResultFactory"/> to return a result.
+        /// </remarks>
+        public abstract SdkResult? Resolve(SdkReference sdkReference,
                                           SdkResolverContext resolverContext,
                                           SdkResultFactory factory);
     }