[5.3] Added check for empty port to prevent Invalid Handler Error (#1…

…6824)

* Added check for empty port.

Prevents DSN from generating with an extra comma and throwing an Invalid Handler Error when port is specified in the database configuration but not set to a value.

Produces 'sqlsrv:Server=localhost;Database=mydb;' instead of 'sqlsrv:Server=localhost,;Database=mydb;'

* Fixed style guide

src/Illuminate/Database/Connectors/SqlServerConnector.php

@@ -155,7 +155,7 @@ protected function buildConnectString($driver, array $arguments)
      */
     protected function buildHostString(array $config, $separator)
     {
-        if (isset($config['port'])) {
+        if (isset($config['port']) && ! empty($config['port'])) {
             return $config['host'].$separator.$config['port'];
         } else {
             return $config['host'];