Nullness library: recognise switches with null checks

java/ql/test/query-tests/Nullness/G.java

@@ -0,0 +1,22 @@
+public class G {
+
+  public static void test(String s) {
+
+    if (s == null) {
+      System.out.println("Is null");
+    }
+
+    switch(s) { // OK; null case means this doesn't throw.
+      case null -> System.out.println("Null");
+      case "foo" -> System.out.println("Foo");
+      default -> System.out.println("Something else");
+    }
+
+    switch(s) { // BAD; lack of a null case means this may throw.
+      case "foo" -> System.out.println("Foo");
+      default -> System.out.println("Something else");
+    }
+
+  }
+
+}