Allow raven.dsn with "null" value

[lyrixx]

See: getsentry/raven-php@62701ba

[lyrixx]

This is not related to this pr but can I make a PR this this kinf of patche:

->validate()
    ->ifTrue(function($v) { return 'raven' === $v['type'] && !class_exists('Raven_Client'); })
    ->thenInvalid('please add "raven/raven" in your composer.json')
->end()

[Seldaek]

The class_exists stuff, I would rather do this sort of check in the Extension than in the Configuration class, but why not add it yes when creating a raven handler it would make sense.

[lyrixx]

Thanks for the quick merge.

Can you tag a new release ?

About the class exist, could this be in monolog itself ?

[stof]

Not really. Monolog expects receiving an instance, so it is too late if you don't have the class.

[lyrixx]

I should read the code before talk ;)