Apply suggestions from code review

Co-authored-by: Johnny Pham <[email protected]>

Author: lcheunglci

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlDependencyListener.cs

@@ -690,10 +690,7 @@ private void Restart(object unused)
                                 }
                                 finally
                                 {
-                                    if (null != context)
-                                    {
-                                        context.Undo();
-                                    }
+                                    context?.Undo();
                                 }
                             }
                             else
@@ -1033,10 +1030,7 @@ private void TearDownAndDispose()
                         _con.Dispose(); // Close and dispose connection.
 #if NETFRAMEWORK
                         //dispose windows identity
-                        if (_windowsIdentity != null)
-                        {
-                            _windowsIdentity.Dispose();
-                        }
+                        _windowsIdentity?.Dispose();
 #endif
                     }
                 }