Fixed ResponseCompression middleware nullability (#28352)

Author: alefranz

src/Middleware/ResponseCompression/src/ResponseCompressionBody.cs

@@ -270,16 +270,14 @@ private void OnWrite()
             }
         }
 
-        [MemberNotNull(nameof(_compressionProvider))]
-        private ICompressionProvider ResolveCompressionProvider()
+        private ICompressionProvider? ResolveCompressionProvider()
         {
             if (!_providerCreated)
             {
                 _providerCreated = true;
                 _compressionProvider = _provider.GetCompressionProvider(_context);
             }
 
-            Debug.Assert(_compressionProvider != null);
             return _compressionProvider;
         }