logger: fix hardfault for invalid SDLOG_PROFILE setting

If some bits are set but no topic was added, _subscriptions was null but
later accessed.

During normal use this only happens when switching between different
firmware versions with different SDLOG_PROFILE definition (with custom
config).

src/modules/logger/logged_topics.cpp

@@ -373,7 +373,7 @@ bool LoggedTopics::initialize_logged_topics(SDLogProfileMask profile)
 		initialize_configured_topics(profile);
 	}
 
-	return true;
+	return _subscriptions.count > 0;
 }
 
 void LoggedTopics::initialize_configured_topics(SDLogProfileMask profile)