Fix notice errors when creating fields named 0.

Fixes #3371

lib/Cake/Test/Case/View/Helper/FormHelperTest.php

@@ -2153,6 +2153,23 @@ public function testInput() {
 		$this->assertTags($result, $expected);
 	}
 
+/**
+ * Test that inputs with 0 can be created.
+ *
+ * @return void
+ */
+	public function testInputZero() {
+		$this->Form->create('User');
+		$result = $this->Form->input('0');
+		$expected = array(
+			'div' => array('class' => 'input text'),
+			'label' => array('for' => 'User0'), '/label',
+			'input' => array('type' => 'text', 'name' => 'data[User][0]', 'id' => 'User0'),
+			'/div'
+		);
+		$this->assertTags($result, $expected);
+	}
+
 /**
  * test input() with checkbox creation
  *

lib/Cake/View/Helper/FormHelper.php

@@ -199,7 +199,7 @@ protected function _introspectModel($model, $key, $field = null) {
 					$this->fieldset[$object->alias]['fields'][$alias] = array('type' => 'multiple');
 				}
 			}
-			if (empty($field)) {
+			if ($field === null || $field === false) {
 				return $this->fieldset[$model]['fields'];
 			} elseif (isset($this->fieldset[$model]['fields'][$field])) {
 				return $this->fieldset[$model]['fields'][$field];
@@ -773,7 +773,7 @@ public function error($field, $text = null, $options = array()) {
  * @link http://book.cakephp.org/2.0/en/core-libraries/helpers/form.html#FormHelper::label
  */
 	public function label($fieldName = null, $text = null, $options = array()) {
-		if (empty($fieldName)) {
+		if ($fieldName === null) {
 			$fieldName = implode('.', $this->entity());
 		}