Polish changes following #3305

Also add support for field comments in PostgreSQL & Oracle ... only via alter_table() for now :/
diff --git a/system/database/drivers/mysqli/mysqli_forge.php b/system/database/drivers/mysqli/mysqli_forge.php
index 38f6249..24dc2b2 100644
--- a/system/database/drivers/mysqli/mysqli_forge.php
+++ b/system/database/drivers/mysqli/mysqli_forge.php
@@ -90,7 +90,7 @@
 	 *
 	 * @var	string
 	 */
-	protected $_null		= 'NULL';
+	protected $_null = 'NULL';
 
 	// --------------------------------------------------------------------
 
@@ -187,8 +187,6 @@
 			$extra_clause = ' FIRST';
 		}
 
-		$comment_clause = isset($field['comment']) ? ' COMMENT ' . $field['comment'] : '';
-
 		return $this->db->escape_identifiers($field['name'])
 			.(empty($field['new_name']) ? '' : ' '.$this->db->escape_identifiers($field['new_name']))
 			.' '.$field['type'].$field['length']
@@ -197,7 +195,7 @@
 			.$field['default']
 			.$field['auto_increment']
 			.$field['unique']
-			.$comment_clause
+			.(empty($field['comment']) ? '' : ' COMMENT '.$field['comment'])
 			.$extra_clause;
 	}