JdbcMessageStore getMessageGroup(Object) doesn't cater for expire-groups-upon-completion = false

when setting the expire-groups-upon-completion="false", if I resend an already processed message it is not discarded. this comes down to the getMessageGroup(Object id) code returning a new SimpleMessageGroup() when the message size == 0 without evaluating if the message group id is still there.

to make the behavior the same as SimpleMessageStore, the size() == 0 logic needs to be removed