[interp] Std.string((null:UInt)) throws

Just run `Std.string((null : UInt))` 👉 `Uncaught exception Invalid operation: null + 0`

(same in nightly `Std.string((null : haxe.UInt32))`)

Since eval is not a static target and the compiler actually allows null value for UInt, I think Std.string should produce `"null"` for it?

Start by reproducing the issue with `Std.string((null : UInt))` in the eval interpreter, then compare it with `Std.string((null : haxe.UInt32))` in nightly. Trace the `Std.string` handling for UInt values; done means both cases return the string `"null"` without throwing an invalid-operation exception.

Written by the indexing model from the issue text.