A fixed version of #22473

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.

Some explicit nulls neg tests have been excluded under the scala2 library tasty tests due to technical reasons, for example:

class S {
  given Conversion[String, Array[String]] = _ => ???

  def f = {
    val s: String | Null = ???

    val x: String = s // error
    val xl = s.length // error
    val xs: Array[String | Null] | Null = s // error
  }
}

val xl = s.length will not give an error. This is because the .length function is from the WrappedString.scala library. After desugaring, it will become wrapString(s).length. Since the scala2 library is compiled without explicit nulls, the types are flexified.wrapString will have argument type FlexibleType(String) and return type FlexibleType(String), therefore it will not give an error. Hence we need to exclude it from the tasty tests.

[test_scala2_library_tasty]