That would suggest that an inexperienced admin has done a really bad thing and added "none" as a resolution.
Go to the list of resolutions and check that! (Also check for any other resolutions that are badly named). If it is, then you'll need to fix it, and as fast as possible. If it's not, then we'll need to look at a totally different pile of stuff
You blank it out, not use a word.
The rule JIRA uses is simple - if you looked in the database, you'll find the table jiraissue has a resolution column. When this is empty, JIRA displays "unresolved". When it's filled (and it doesn't matter what with), then an issue is struck out, and treated as resolved.
If you just delete none, JIRA will make you migrate it to another resolution. You might not want to do that, so start by renaming it to "broken resolution, do not use" or something.
Then run a report on how big the problem is - a quick search for "resolution = broken" - what does that look like?
Nic's right, when you identify the issues that need to have their resolution reset, you can follow the steps in the following link to clean things up. (been there, done that!)
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