Both the Ender acid (the one that comes from the ender charges) and the Ender spit (the one that comes directly from the dragon when it's in the middle of the exit gateway) deal no damage, no knockback, and produce particles infinitely. Right-clicking / using an empty bottle allows you to collect an infinite amount of Dragon Breath as long as you have bottles.

I'd like to note, before 1.9 the dragon breath actually did too much damage and too much knockback. Now it's literally the opposite. The End fight as a whole is really really broken, it's kinda disheartening to see major things like these not being taken care of whenever a major update comes out. That being said, I really appreciate the effort put into making this game as polished as possible. Thank you.