fix: default value for imap folder

Author: nicodh

src/login_param.rs

@@ -70,6 +70,7 @@ pub struct EnteredImapLoginParam {
     /// Folder to watch.
     ///
     /// If empty, user has not entered anything and it shuold expand to "INBOX" later.
+    #[serde(default)]
     pub folder: String,
 
     /// Socket security.