check for Null<UInt> null check and toString()

[ncannasse]

This is adds some checks for #6760 regarding Null<UInt> handling.

For comparison, defining a function body was triggering a Null<UInt> to UInt cast. However we don't currently have a solution for toString() which requires a function body but will then force the cast and prevent correctly printing "null".

This might affect other platforms than HL (Travis will tell us).

[Simn]

Updated the branch, this fails on JVM and C++ targets with line: 14, expected 0 but it is 10.