[plugin] Exporter: fix crash on joplin when token is invalid (#9203)

https://github.com/koreader/koreader/issues/9197#issuecomment-1153129147

plugins/exporter.koplugin/target/joplin.lua

@@ -124,6 +124,7 @@ function JoplinExporter:notebookExist(title)
     local response, err = makeRequest(url, "GET")
     if not response then
         logger.warn("Joplin notebookExist error", err)
+        return false
     end
 
     if not response.items or type(response.items) ~= "table" then