Fix codegen for closures returning 'Unit?'

Only do a checkcast when we need to coerce Object to Unit: the code was
hopefully type-checked so that it'll only be necessary when the value is either
Unit.VALUE or null

compiler/testData/codegen/box/unit/nullableUnitInWhen3.kt

@@ -0,0 +1,12 @@
+fun foo() {}
+
+fun box(): String {
+    val x = when ("A") {
+        "B" -> foo()
+        else -> null
+    }
+
+    foo()
+    
+    return if (x == null) "OK" else "Fail"
+}