Zero mixins were disallowed years ago with some very limited exceptions (for spending old coins). Initially the minimum ring size was 3 but is now 5. You can see from the stats that they essentially never happen any more: https://moneroblocks.info/stats/ring-size

Source: https://www.reddit.com/r/Monero/comments/7sv5vv/new_to_monero_zeromixintxs_already_fixed/dt7phdw/