ILSpySymbolSource.GetLocalVariables: in case of error, return an empty collection instead of null

src/AddIns/DisplayBindings/ILSpyAddIn/ILSpySymbolSource.cs

@@ -96,7 +96,7 @@ namespace ICSharpCode.ILSpyAddIn
 			var file = GetSymbols(method);
 			
 			if (file == null || !file.DebugSymbols.ContainsKey(id))
-				return null;
+				return Enumerable.Empty<ILLocalVariable>();
 			
 			var symbols = file.DebugSymbols[id];