I got curious and I checked; there have been a few one-character submissions so it’s not the shortest.

     ⊥: https://qht.co/item?id=7530548
     {: https://qht.co/item?id=7729399
     M: https://qht.co/item?id=4367006

In total (at least in the [fh-bigquery:hackernews.stories] BQ dataset), there have been 15 non-deleted submissions with a one-character title.