Fix the get_torrent_health endpoint in GUI tests

[kozlovsky]

Currently, the RESTComponent.run method has the following code lines:

        torrent_checker = None if config.gui_test_mode else torrent_checker_component.torrent_checker
        tunnel_community = None if config.gui_test_mode else tunnel_component.community
        gigachannel_manager = None if config.gui_test_mode else gigachannel_manager_component.gigachannel_manager

and

        self.maybe_add('/metadata', MetadataEndpoint, torrent_checker, metadata_store_component.mds,
                       knowledge_db=knowledge_component.knowledge_db,
                       tag_rules_processor=knowledge_component.rules_processor)

That is, under GUI tests, the torrent_checker argument of MetadataEndpoint can be None. The maybe_add call does not handle this situation, as it can only handle NoneComponent arguments and not the normal None values.

But, the MetadataEndpoint.get_torrent_health methods expect self.torrent_checker to be not-None.

For some reason, during GUI tests, the get_torrent_health endpoint can be randomly called in different GUI tests, provoking a Core exception and a test failure.

This PR fixes the get_torrent_health endpoint by handling the case when self.torrent_checker is None.

[drew2a]

Related to #7498