Making modParams ignore null results.

Test added.

cake/libs/object_collection.php

@@ -73,7 +73,10 @@ abstract public function load($name, $options = array());
  *    objects. Defaults to false.
  *
  * - `modParams` Allows each object the callback gets called on to modify the parameters to the next object.
- *    Setting modParams to an integer value will allow you to modify the parameter with that index. Defaults to false.
+ *    Setting modParams to an integer value will allow you to modify the parameter with that index.
+ *    Any non-null value will modify the parameter index indicated.
+ *    Defaults to false.
+ *   
  * 
  * @param string $callback Method to fire on all the objects. Its assumed all the objects implement
  *   the method you are calling.
@@ -115,7 +118,7 @@ public function trigger($callback, $params = array(), $options = array()) {
 				(is_array($options['breakOn']) && in_array($result, $options['breakOn'], true)))
 			) {
 				break;
-			} elseif ($options['modParams'] !== false) {
+			} elseif ($options['modParams'] !== false && $result !== null) {
 				$params[$options['modParams']] = $result;
 			}
 		}

cake/tests/cases/libs/object_collection.test.php

@@ -315,6 +315,35 @@ function testTriggerModParamsInvalidIndex() {
 		);
 	}
 
+/**
+ * test that returrning null doesn't modify parameters.
+ *
+ * @expectedException CakeException
+ * @return void
+ */
+	function testTriggerModParamsNullIgnored() {
+		$this->_makeMockClasses();
+		$this->Objects->load('TriggerMockFirst');
+		$this->Objects->load('TriggerMockSecond');
+
+		$this->Objects->TriggerMockFirst->expects($this->once())
+			->method('callback')
+			->with(array('value'))
+			->will($this->returnValue(null));
+
+		$this->Objects->TriggerMockSecond->expects($this->once())
+			->method('callback')
+			->with(array('value'))
+			->will($this->returnValue('new value'));
+
+		$result = $this->Objects->trigger(
+			'callback',
+			array(array('value')),
+			array('modParams' => 2)
+		);
+		$this->assertEquals('new value', $result);
+	}
+
 /**
  * test normalizeObjectArray
  *