Strip Nulls from Java members when unsafeNulls is enabled

[noti0na1]

The unsafeNulls feature makes Null a subtype of AnyRef in a certain scope.
In the scope, we can assign a nullable value with non-nullable type, or select members on it without .nn.

However, unsafeNulls still couldn't handle Java methods or fields if the type is a value type or a type variables.

J.java

public class J {
  public <T> T g() {
    return null;
  }
}

S.scala

import scala.language.unsafeNulls

val j: J = new J

// After JavaNullInterop, the signature of `J.g` becomes: `def g[T]: T | Null`

val s: String = j.g[String]() // ok
// because `String | Null` is not a subtype of `String`

val i: Int = j.g[Int]() // error
// the type of `g[Int]()` is `Int | Null`
// `Int | Null` is not a subtype of `Int` even if `Null` a subtype of `AnyRef`

def col[T]: T = j.g[T]() // error
// similarly,  we can't say `T| Null` is a subtype of `T`

The solution is to cast all Java applications and selections to non-nullable in adaptToSubtype when unsafeNulls is enabled. This will not affect Scala applications and selections.

The new test is in tests/explicit-nulls/unsafe-common/unsafe-java-call/

[noti0na1]

Hi @odersky , the PR passes all tests now. Could you review it?

[odersky]

LGTM!