Add test case for unapply on zero arity case class

Author: OlivierBlanvillain

tests/run/zero-arity-case-class.scala

@@ -0,0 +1,26 @@
+case class Foo()
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert(Foo.unapply(Foo()) == true)
+
+    // unapply generate by scalac are `_ != null`,
+    // dotty returns true in all cases
+    assert(Foo.unapply(null) == true)
+
+    Foo() match {
+      case Foo() => ()
+      case _     => ???
+    }
+
+    Foo() match {
+      case _: Foo => ()
+      case _      => ???
+    }
+
+    (Foo(): Any) match {
+      case Foo() => ()
+      case _     => ???
+    }
+  }
+}