change accessor to return null not throw with no factory

[johnataylor]

fixes #1191

Note this is actually a change of behavior - but it is closer to the JavaScript.

Not exactly the same because JavaScript has the notion of undefined - and null is not the same. This is particularly evident if the accessor model is being used to access raw properties - as is done in the unit tests.

As the conversation in the issue suggests, its very hard to imagine this being used in this way out side of the immediate context of the dialog system where currently it must be passed in. And as other issues have covered, even in that context it is only ever used in a one particular way (as a ref and so only Get is called.)

And the change is safe for the usage in the Dialog system.

[coveralls]

Pull Request Test Coverage Report for Build 47938

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.97%

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Totals
Change from base Build 47934:	0.0%
Covered Lines:	3408
Relevant Lines:	5014

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💛 - Coveralls