Report error about null value for any type that cannot be null, not for only built-in

[atzedus]

Validator skip any checks when validating custom scalar types.
But if any type marked as not nullable ('Time!' for example) it doesn't matter what type is it custom or basic when provided 'null' value.

[coveralls]

Coverage increased (+0.002%) to 92.287% when pulling 2e01e2d on atzedus:master into d3d9eb0 on vektah:master.

[StevenACoffman]

Thanks!