Minor, add more tests on signature-polymorphic calls

Add a test on null (since null is a special case in the
PolymorphicSignature spec), and a test on a call without assignment to a
variable

compiler/testData/codegen/box/polymorphicSignature/nullArgument.kt

@@ -0,0 +1,28 @@
+// !LANGUAGE: +PolymorphicSignature
+// TARGET_BACKEND: JVM
+// IGNORE_BACKEND_FIR: JVM_IR
+// FULL_JDK
+// SKIP_JDK6
+// WITH_RUNTIME
+
+import java.lang.invoke.MethodHandles
+import java.lang.invoke.MethodType
+
+var result: String? = "Fail"
+
+class C {
+    fun foo(s: Nothing?) {
+        result = s
+    }
+}
+
+fun box(): String {
+    val mh = MethodHandles.lookup().findVirtual(
+        C::class.java, "foo",
+        MethodType.methodType(Void.TYPE, Void::class.java)
+    )
+
+    mh.invokeExact(C(), null)
+
+    return result ?: "OK"
+}