Update join building to not error out on empty conditions.

When conditions are empty we can assume one of two things:

* The person made a mistake.
* The person is doing the join conditions in the where clause.

In both cases we should attempt to generate proper SQL.

Fixes #4189

lib/Cake/Model/Datasource/DboSource.php

@@ -1865,7 +1865,7 @@ class DboSource extends DataSource {
 			'type' => null,
 			'alias' => null,
 			'table' => 'join_table',
-			'conditions' => array()
+			'conditions' => '',
 		), $join);
 
 		if (!empty($data['alias'])) {
@@ -1919,7 +1919,7 @@ class DboSource extends DataSource {
  * @return string
  */
 	public function renderJoinStatement($data) {
-		if (strtoupper($data['type']) === 'CROSS') {
+		if (strtoupper($data['type']) === 'CROSS' || empty($data['conditions'])) {
 			return "{$data['type']} JOIN {$data['table']} {$data['alias']}";
 		}
 		return trim("{$data['type']} JOIN {$data['table']} {$data['alias']} ON ({$data['conditions']})");

lib/Cake/Test/Case/Model/Datasource/DboSourceTest.php

@@ -1201,6 +1201,11 @@ class DboSourceTest extends CakeTestCase {
 				'type' => 'LEFT',
 				'alias' => 'PostsTag',
 				'table' => 'posts_tags',
+			), 'LEFT JOIN cakephp.posts_tags AS PostsTag'),
+			array(array(
+				'type' => 'LEFT',
+				'alias' => 'PostsTag',
+				'table' => 'posts_tags',
 				'conditions' => array('PostsTag.post_id = Post.id')
 			), 'LEFT JOIN cakephp.posts_tags AS PostsTag ON (PostsTag.post_id = Post.id)'),
 			array(array(