[cs] don't disable the whole TestCSharp

disable only the failing test and add a TODO with a note why it was
disabled
[cs] fix comparison of Null<T> with null constant (closes #2474)

add a test for it to ensure it won't be broken
Merge pull request #2515 from nadako/2474_null_comparison

[cs] fix comparison of haxe.lang.Null with null constant