Fixed D-Bus method get_tab_name which could return illegal NoneType value instead of string

When you first create a new tab in Guake, the `get_window_title()` method may return `None` while it is still loading a bash shell -- after which it works as expected and returns the string tab name.

Therefore, in the intervening period after creating a new terminal tab and before the tab finishes loading, this exception is raised in the D-Bus service after our dbusiface.py passes a `NoneType` instead of a string:

	Gio.IOErrorEnum: GDBus.Error:org.freedesktop.DBus.Python.TypeError: Traceback (most recent call last):
	  File /usr/lib/python2.7/dist-packages/dbus/service.py, line 751, in _message_cb
	    _method_reply_return(connection, message, method_name, signature, *retval)
	  File /usr/lib/python2.7/dist-packages/dbus/service.py, line 254, in _method_reply_return
	    reply.append(signature=signature, *retval)
	TypeError: Expected a string or unicode object

This pull request merely returns an empty string `''` if it receives a `NoneType` from `get_window_title()`.