Don't throw an exception when the handlebars template reference an not existing property

[yannisgu]

At the moment when a handlbars template uses a not existing property, Veil will crash.
So when I have the following expression:

{{myProp}}

But there is no {{myProp}} Veil will throw an Exception ("Unable to parse model expression", https://github.com/csainty/Veil/blob/master/src/Veil.Handlebars/HandlebarsExpressionParser.cs#L57).
In our case it would be much better if this expression would be treated as null value.
Why? We develop the handlbars templates and the according models are developed by different people. We want that properties can be added to the handlbars template, but added later to the model, without to break our mainline.

Therefor I propose to change this, so that when a member doesn't exist, Veil treats it as null-String.

In order to do this we would need to make the following additions to Veil:

• https://github.com/csainty/Veil/blob/master/src/Veil.Handlebars/HandlebarsExpressionParser.cs#L57
  Return an SyntaxTreeExpression.Empty() (new class) instead of throwing the exception
• https://github.com/csainty/Veil/blob/master/src/Veil/Compiler/VeilTemplateCompiler.ParseExpression.cs#L20
  Adding an if for our new EmptyExpressionNode
  In this case ParseExpression should return Expression.Constant(null, typeof(string))

What do you think?

[csainty]

I can see the value in this, though I am not sure I would want that as standard behaviour. It will silently hide typos in the templates which will be hard to track down.

Made configurable though, I see no reason not to support it.

[yannisgu]

Cool - what would be your favored way to configure this? Constructor ofVeilEngine?