Just a quick suggestion based on this discussion: http://forum.osticket.com/d/discussion//resolved-can-t-upgrade-from-1-9-3-to-1-9-4#latest
Would be useful to have a check if the files are missing or existing. Maybe based on the database config, e.g. IF config for plugins exists in the database and is active field in db is set to 1, check whether the files are there and otherwise skip the check since no plugins seems to be used/installed or disabled.
Btw. I ran into the same issue some time ago while I tested an upgrade and instead of replacing the files, I extracted the new files to a different directory and forgot to add the plugin files. Stupid, but with better checks this issue doesn't need to happen again in future releases ;)
Greetings,
Michael