Null interception in PrettyPrinter

[kindermannhubert]

@waf what is the reasoning behind this?

csi.exe and Immediate window in VS also outputs null text and I would probably prefer it too.

This could surely be configurable but I'm not sure if it's necessary.

[waf]

@kindermannhubert the reasoning is to avoid the following behavior, where assignments (e.g. var x = 5;) return a null object from the Roslyn APIs:

I agree that the behavior in csi.exe is better here, where it doesn't display null for assignments, but does display in cases where the object being evaluated is null -- I believe back when this was written (a while ago now!) I didn't see a way of distinguishing the two cases in the script evaluation API.

[kindermannhubert]

Indeed Roslyn provides no information on whether the input is void-returning/no-output or value-returning. I've prepared a custom analysis of this in #187.