[4.2] Fixed empty string in database connection config 'unix_socket'

Causes system to default to socket connection instead of host connection. The intention of someone passing an empty string as a socket value would be to not use a socket. The default config includes this value, and in previous versions you could safely ignore it.

Issue described here: http://stackoverflow.com/questions/25035135/laravel-failing-to-connect-to-db-after-4-2-upgrade/25035973