Fix whenexpr test

Prior to Kotlin 1.7 the gratuitous `?` was ignored for typing purposes; now it yields a `String?`. We should make the test work everywhere by using a real nullable type.

java/ql/test/kotlin/library-tests/dataflow/whenexpr/WhenExpr.kt

@@ -1,10 +1,10 @@
 class WhenExpr {
   fun taint() = Uri()
 
-  fun sink(s: String) { }
+  fun sink(s: String?) { }
 
-  fun bad() {
-    val s0 = taint()
+  fun bad(b: Boolean) {
+    val s0 = if (b) taint() else null
     sink(s0?.getQueryParameter())
   }
 }

java/ql/test/kotlin/library-tests/dataflow/whenexpr/test.expected

@@ -1 +1 @@
-| WhenExpr.kt:7:14:7:20 | taint(...) | WhenExpr.kt:8:14:8:32 | <Stmt> |
+| WhenExpr.kt:7:21:7:27 | taint(...) | WhenExpr.kt:8:14:8:32 | <Stmt> |