(TK-22) Fix for Jetty9 service shutdown error when not first inited

This commit adds a simple check in the Jetty9 service's `stop` function
which avoids calling the `core/shutdown` function if no `:jetty9-server`
is attached to the context.  This avoids a schema violation `Exception`
which would otherwise be thrown for any case where the Jetty9 service's
`stop` function were called without the Jetty9 service's `init` function
first being called.  The prior `Exception` would have only occurred if
another service had thrown a `Throwable` from its `init` function and the
Jetty9 service's `init` function had not already been called.
Merge pull request #33 from camlow325/bug/master/TK-22-jetty-errors-during-shutdown-if-not-inited

(TK-22) Fix for Jetty9 service shutdown error when not first inited