Allow to connect without a dbname param

The PDO connection allow the instantiation of the connection even if the dbname
parameter isn't present. This change is there to make sure both behave the same way.

lib/Doctrine/DBAL/Driver/Mysqli/MysqliConnection.php

@@ -51,6 +51,7 @@ class MysqliConnection implements Connection, PingableConnection, ServerInfoAwar
     {
         $port = isset($params['port']) ? $params['port'] : ini_get('mysqli.default_port');
         $socket = isset($params['unix_socket']) ? $params['unix_socket'] : ini_get('mysqli.default_socket');
+        $dbname = isset($params['dbname']) ? $params['dbname'] : null;
 
         $flags = isset($driverOptions[static::OPTION_FLAGS]) ? $driverOptions[static::OPTION_FLAGS] : null;
 
@@ -61,7 +62,7 @@ class MysqliConnection implements Connection, PingableConnection, ServerInfoAwar
         $previousHandler = set_error_handler(function () {
         });
 
-        if ( ! $this->_conn->real_connect($params['host'], $username, $password, $params['dbname'], $port, $socket, $flags)) {
+        if ( ! $this->_conn->real_connect($params['host'], $username, $password, $dbname, $port, $socket, $flags)) {
             set_error_handler($previousHandler);
 
             $sqlState = 'HY000';