A slightly humorous thought occurred to me (a recovering rules lawyer) after reading this question and answers.
"None of the answers actually specify that 0 is the minimum damage."
Logically it wouldn't make sense for an attack to apply negative damage or healing, except in specific cases like hitting a flame elemental with a fire attack. Ruling that you could do negative damage would lead to an exploit to avoid costs of healing. As such I highly doubt any DM would make such a ruling. I definitely wouldn't except for the most absurd and humorous of games were failure is the expected result not success.
Another possibility (as suggested by Secespitus) is that negative damage would reflect back on the attacker. This could be seen as punching a strongman so hard (or in just the wrong way) as to injure your own hand. This would also play well in a failure is the only option game, and if there is a rule suggesting that it would be an appropriate answer to this question as well.
Take the case of a person with less than 8 Strength, no levels in monk, and does not have a tavern brawler. Would they do -1(1 - 2) damage on unarmed strikes? If so what would the result of the negative damage be?
- "Healing" the target because such a puny hit actually raises their fighting spirits and makes them more capable of fighting
- Reflective Damage, the attacker actually takes 1 point of damage, representing injury to his hand or other damage
I am curious to know if there is an official rule stating that the minimum damage an attack can do is 0 or is there something in the damage calculation that I am missing that would otherwise prevent negative damage?