Allow case null, default to be the first switch case

This is consistent with existing treatment of `case null: default:`

Author: smowton

java/ql/lib/semmle/code/java/ControlFlowGraph.qll

@@ -471,6 +471,7 @@ private module ControlFlowGraphImpl {
   private SwitchCase getASuccessorSwitchCase(PatternCase pred) {
     result.getParent() = pred.getParent() and
     result.getIndex() > pred.getIndex() and
+    // Note we do include `case null, default` (as well as plain old `default`) here.
     not result.(ConstCase).getValue(_) instanceof NullLiteral and
     (
       result.getIndex() <= getNextPatternCase(pred).getIndex()
@@ -492,6 +493,8 @@ private module ControlFlowGraphImpl {
     (
       result.(ConstCase).getValue(_) instanceof NullLiteral
       or
+      result instanceof NullDefaultCase
+      or
       not exists(getPatternCase(switch, _))
       or
       result.getIndex() <= getPatternCase(switch, 0).getIndex()

java/ql/test/library-tests/pattern-switch/cfg/test.expected

@@ -144,6 +144,7 @@
 | Test.java:55:13:55:25 | (...)... | Test.java:56:8:56:21 | case ... |
 | Test.java:55:13:55:25 | (...)... | Test.java:58:8:58:21 | case ... |
 | Test.java:55:13:55:25 | (...)... | Test.java:61:8:61:42 | case T t ... |
+| Test.java:55:13:55:25 | (...)... | Test.java:69:8:69:26 | case null, default |
 | Test.java:55:21:55:25 | thing | Test.java:55:13:55:25 | (...)... |
 | Test.java:56:8:56:21 | case ... | Test.java:57:10:57:44 | <Expr>; |
 | Test.java:57:10:57:19 | System.out | Test.java:57:29:57:42 | "It's Const1!" |