Make "no description" really mean "no description"

Gitter is a library for accessing git repositories from PHP.
It's meant to be used by other (higher-level) tools to provide
that functionality.

Having Gitter talk to the user by means of fake description messages
seems wrong.

This makes not having a description file for a repository
cause the description to be NULL.
Some higher-level tool that uses this library can then detect this
NULL value and decide how to report it to the user.
Merge pull request #19 from spantaleev/null-description

Make "no description" really mean "no description"