Fix passing Seq or Array as a repeated argument when its type is union type

[noti0na1]

In the following example, the type of s is Seq[String] | Null, which is equivalent to Seq[String]. Hence, we should be allowed to pass it as a repeated argument.

import java.nio.file.Paths
def f = {
  val s: Seq[String] | Null = null
  Paths.get("", s: _*)
}

Without this fix, we will get a Type Mismatch Error:

Found:    Seq[String]
Required: String*

[noti0na1]

@smarter I add a special case to elemTpe and derivesFrom to handle T | Null or T | Nothing. I also rename the isBottomType to isNothingType in Types.