Apply Flexible Types to Symbols from files compiled without Explicit Nulls

[HarrisL2]

This PR wraps the types of symbols from files compiled without explicit nulls in flexible types.
This allows for interop between multiple files in cases where

class Unsafe_1 {
  def foo(s: String): String = {
    if (s == null) then "nullString"
    else s
  }
}

compiled without explicit nulls can still be used in

def Flexible_2() =
  val s2: String | Null = "foo"
  val unsafe = new Unsafe_1()
  val s: String = unsafe.foo(s2)
  unsafe.foo("")
  unsafe.foo(null)

whereas the argument would have been a strictly non-null String, because of the flexible type, the function call is now permitted.

[olhotak]

This LGTM, but @noti0na1 can you also have a look?

[noti0na1]

LGTM