fix(query_row_optional): do not treat rows with NULL as missing rows

Instead of treating NULL type error
as absence of the row,
handle NULL values with SQL.
Previously we sometimes
accidentally treated a single column
being NULL as the lack of the whole row.

src/mimefactory.rs

@@ -201,7 +201,8 @@ impl MimeFactory {
         let (in_reply_to, references) = context
             .sql
             .query_row(
-                "SELECT mime_in_reply_to, mime_references FROM msgs WHERE id=?",
+                "SELECT mime_in_reply_to, IFNULL(mime_references, '')
+                 FROM msgs WHERE id=?",
                 (msg.id,),
                 |row| {
                     let in_reply_to: String = row.get(0)?;