null.asInstanceOf[T] where T is a value type results in NullPointerException

While the direct route of

```scala
case class Foo(d: Double) extends AnyVal
null.asInstanceOf[Foo]
```

was fixed in #5866, the issue remains when Generics get involved:

```scala
scala> case class Foo(d: Double) extends AnyVal
defined class Foo

scala> def newInst[T] = null.asInstanceOf[T]
newInst: [T]=> T

scala> newInst[Foo]
java.lang.NullPointerException
  ... 32 elided
```