Fix type NamedArg of annotations when pt is nullable

[noti0na1]

Thie PR fixs typing NamedArg of annotations when pt is nullable.

scalac -Yexplicit-nulls Jtest.java

public class Jtest {
  @SuppressWarnings("unused")
  public void f() {}
}

Without the fix:

-- [E007] Type Mismatch Error: Jtest.java:2:19 ---------------------------------
2 |  @SuppressWarnings("unused")
  |                   ^
  |                   Found:    ("SuppressWarnings" : String)
  |                   Required: scala.Array[String | scala.Null] | scala.Null

Since the argument type of the annotation is nullable under explicit nulls, we need to strip the Null type first in order to try the Array transformation.