sentry: Do not initialize the sentry SDK if sentry_dsn config is missing

It seems useless to setup sentry if no sentry server is available, plus
it will prevent a spurious error message when used in gunicorn;
sentry_sdk will try to identify a git hash for the running code, which
will generate an error from the git suprocess, which will be caught by
gunicorn's Arbiter.reap_workers() method, which will show an error
message like:

  [ERROR] Worker (pid:60) exited with code 128