Fix IfElseIfConstructToSwitch null safety regression (#1014)

* Fix IfElseIfConstructToSwitch null safety regression (issue #1013)

When converting instanceof chains to switch without an explicit null check,
emit `case null, default` instead of just `default` to preserve instanceof's
null-safe behavior. Previously, the recipe would generate code that throws NPE
when the switched variable is null, unlike the original if-else chain.

* Fix UpgradeToJava21Test expectation for case null, default

Author: timtebeek

src/main/java/org/openrewrite/java/migrate/lang/IfElseIfConstructToSwitch.java

@@ -212,7 +212,7 @@ public J.Return visitReturn(J.Return return_, AtomicBoolean atomicBoolean) {
                 arguments[i++] = getPattern(instanceOf);
                 arguments[i++] = getStatement(entry.getValue());
             }
-            switchBody.append("default -> #{any()};\n");
+            switchBody.append(nullCheckedParameter != null ? "default" : "case null, default").append(" -> #{any()};\n");
             if (else_ != null) {
                 arguments[i] = getStatement(else_);
             } else {

src/test/java/org/openrewrite/java/migrate/UpgradeToJava21Test.java

@@ -61,7 +61,7 @@ private static double convertToDouble(Object value) {
                             case String string -> Double.parseDouble(string);
                             case Integer integer -> integer.doubleValue();
                             case Long long1 -> long1.doubleValue();
-                            default -> (double) value;
+                            case null, default -> (double) value;
                         };
                     }
                 }

src/test/java/org/openrewrite/java/migrate/lang/IfElseIfConstructToSwitchTest.java

@@ -128,7 +128,7 @@ static String formatter(Object obj) {
                               String str = "String";
                               formatted = String.format("%s %s", str, s);
                           }
-                          default -> formatted = "unknown";
+                          case null, default -> formatted = "unknown";
                       }
                       return formatted;
                   }
@@ -331,7 +331,7 @@ static String formatter(Object obj) {
                               String str = "String";
                               formatted = String.format("%s %s", str, s);
                           }
-                          default -> formatted = "unknown";
+                          case null, default -> formatted = "unknown";
                       }
                       return formatted;
                   }
@@ -436,7 +436,7 @@ static String describe(Object obj) {
                           case Dog d -> result = "dog";
                           case Cat c -> result = "cat";
                           case Bird b -> result = "bird";
-                          default -> result = "unknown";
+                          case null, default -> result = "unknown";
                       }
                       return result;
                   }