Use IsNull instead of reference equality comparision

ExpressionBody is NullExpression when absent.

ICSharpCode.Decompiler/CSharp/SequencePointBuilder.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2017 Daniel Grunwald
+// Copyright (c) 2017 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -211,7 +211,7 @@ public override void VisitLambdaExpression(LambdaExpression lambdaExpression)
 
 		public override void VisitPropertyDeclaration(PropertyDeclaration propertyDeclaration)
 		{
-			if (propertyDeclaration.ExpressionBody != null)
+			if (!propertyDeclaration.ExpressionBody.IsNull)
 			{
 				VisitAsSequencePoint(propertyDeclaration.ExpressionBody);
 			}
@@ -223,7 +223,7 @@ public override void VisitPropertyDeclaration(PropertyDeclaration propertyDeclar
 
 		public override void VisitIndexerDeclaration(IndexerDeclaration indexerDeclaration)
 		{
-			if (indexerDeclaration.ExpressionBody != null)
+			if (!indexerDeclaration.ExpressionBody.IsNull)
 			{
 				VisitAsSequencePoint(indexerDeclaration.ExpressionBody);
 			}