Fix bug in \Orm\Model_Temporal::find_revision() throwing exceptions when...

... no revision was found

This PR fixes exceptions being thrown when no revision can be found on the given model. It resulted from the query result being ```null``` when nothing was found and then calling ```set_lazy_timestamp``` on a non-object would throw a according exception.

Signed-off-by: Philipp Tempel <phtempel@gmail.com>