[typer] Monomorph vs Null<T> inference

[kLabz]

This is a regression introduced in 4.3.0 with https://github.com/HaxeFoundation/haxe/commit/02a7f9834b1327507eb9b6f646697960a0ec657f

This can also lead to Fatal error: exception Invalid_argument("List.map2") in genhl with a slightly different example. Also allows this kind of things https://try.haxe.org/#05A1D90d

4.2.5

Main.hx:12: characters 9-12 : Warning : Unknown<0> : { doWithBar : () -> Unknown<1> }
Main.hx:13: characters 9-22 : Warning : () -> Unknown<0>
Main.hx:16: characters 9-12 : Warning : Unknown<0> : { doWithBar : () -> Unknown<1> }
Main.hx:17: characters 9-22 : Warning : () -> Unknown<0>
Main.hx:6: characters 35-38 : error: (?bar : Null<Bar>) -> Void should be () -> Unknown<0>
Main.hx:6: characters 35-38 : ... have: { doWithBar: (?...) -> ... }
Main.hx:6: characters 35-38 : ... want: { doWithBar: () -> ... }
Main.hx:6: characters 35-38 : ... For function argument 'foo'

Since 4.3.0

Notice how we lose the doWithBar typing:

Main.hx:12: characters 9-12 : Warning : Unknown<0> : { doWithBar : () -> Unknown<1> }
Main.hx:13: characters 9-22 : Warning : () -> Unknown<0>
Main.hx:16: characters 9-12 : Warning : Null<Unknown<0>>
Main.hx:17: characters 9-22 : Warning : Unknown<0>
Main.hx:11: characters 3-16 : Don't know how to cast (Bar):void to ():dyn

With this PR

[WARNING] Main.hx:12: characters 9-12

 12 |   $type(foo);
    |         ^^^
    | Unknown<0> : { doWithBar : () -> Unknown<1> }

[WARNING] Main.hx:13: characters 9-22

 13 |   $type(foo.doWithBar);
    |         ^^^^^^^^^^^^^
    | () -> Unknown<0>

[WARNING] Main.hx:16: characters 9-12

 16 |   $type(foo);
    |         ^^^
    | Null<{ doWithBar : () -> Unknown<0> }>

[WARNING] Main.hx:17: characters 9-22

 17 |   $type(foo.doWithBar);
    |         ^^^^^^^^^^^^^
    | () -> Unknown<0>

[ERROR] Main.hx:6: characters 35-38

  6 |   doThings = (foo -> doThingsImpl(foo));
    |                                   ^^^
    | error: (?bar : Null<Bar>) -> Void should be () -> Unknown<0>
    | have: { doWithBar: (?...) -> ... }
    | want: { doWithBar: () -> ... }