[null-safety] False positives on Python

[sebthom]

The following code works fine on all targets except Python where it throws Null safety: Cannot assign nullable value here.

https://try.haxe.org/#da6123e9

@:nullSafety
class Test {
  static function main() {
      #if sys
         ~/win/i.match(Sys.systemName()); // <-- fails to compile
      #end

      var right:Null<Dynamic> = ~/win/i;
      if (Std.isOfType(right, EReg)) {
         cast(right, EReg).match("Windows"); // <-- fails to compile
      }
  }
}