Allow null fallback values to be passed through

Fixes #267

Author: jhalterman

src/main/java/net/jodah/failsafe/ExecutionResult.java

@@ -118,11 +118,13 @@ ExecutionResult withNonResult() {
    * {@code this} if {@link #success} and {@link #result} are unchanged.
    */
   public ExecutionResult withResult(Object result) {
-    return success && ((this.result == null && result == null) || (this.result != null && this.result.equals(result))) ?
+    boolean unchangedNull = this.result == null && result == null && failure == null;
+    boolean unchangedNotNull = this.result != null && this.result.equals(result);
+    return success && (unchangedNull || unchangedNotNull) ?
       this :
       new ExecutionResult(result, null, nonResult, waitNanos, true, true, successAll);
   }
-  
+
   /**
    * Returns a copy of the ExecutionResult with {@code complete} set to false, else this if nothing has changed.
    */

src/test/java/net/jodah/failsafe/issues/Issue267Test.java

@@ -0,0 +1,40 @@
+package net.jodah.failsafe.issues;
+
+import net.jodah.failsafe.Failsafe;
+import net.jodah.failsafe.Fallback;
+import net.jodah.failsafe.Timeout;
+import net.jodah.failsafe.event.ExecutionAttemptedEvent;
+import org.testng.annotations.Test;
+
+import java.net.ConnectException;
+import java.time.Duration;
+
+import static org.testng.Assert.assertNull;
+
+@Test
+public class Issue267Test {
+  public void test() {
+    Timeout<Object> timeout = Timeout.of(Duration.ofMillis(1000L));
+    Fallback<Object> notFoundFallback = Fallback.of(this::handleNotFound).handleIf(this::causedBy404);
+    Fallback<Object> failureHandling = Fallback.ofException(this::handleException);
+
+    Integer result = Failsafe.with(failureHandling, notFoundFallback, timeout).get(this::connect);
+    assertNull(result);
+  }
+
+  private Integer connect() throws ConnectException {
+    throw new ConnectException();
+  }
+
+  private boolean causedBy404(Object o, Throwable throwable) {
+    return throwable instanceof ConnectException;
+  }
+
+  private Object handleNotFound(ExecutionAttemptedEvent<?> event) {
+    return null;
+  }
+
+  private Exception handleException(ExecutionAttemptedEvent<?> event) {
+    return new IllegalArgumentException();
+  }
+}