Skip IfElseIfConstructToSwitch when a trailing non-== null binary check is present (#1109)

Previously the trailing branch's body was silently dropped because `validatePotentialCandidate()` overwrote the disqualifying flag set by `handleNullCheck` when it could not recognise the condition.

Fixes #1108

Author: timtebeek

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

@@ -118,6 +118,9 @@ private SwitchCandidate(J.If if_, Cursor cursor) {
                     return;
                 }
             }
+            if (!potentialCandidate) {
+                return;
+            }
             potentialCandidate = validatePotentialCandidate();
         }
 

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

@@ -904,6 +904,32 @@ else if (obj instanceof Long l) {
             );
         }
 
+        @Test
+        void noSwitchBlockWithTrailingNonEqualsBinaryCheck() {
+            // https://github.com/openrewrite/rewrite-migrate-java/issues/1108
+            // The trailing `else if (o != null)` is not a recognised null check and must not be silently dropped.
+            rewriteRun(
+              //language=java
+              java(
+                """
+                  public class A {
+                      void test(Object o) {
+                          if (o instanceof String s) {
+                              System.out.println(s);
+                          } else if (o instanceof Integer i) {
+                              System.out.println(i);
+                          } else if (o instanceof Boolean b) {
+                              System.out.println(b);
+                          } else if (o != null) {
+                              System.out.println(o);
+                          }
+                      }
+                  }
+                  """
+              )
+            );
+        }
+
         @Test
         void noSwitchBlockWhenElseBlockReturns() {
             rewriteRun(