Yes -a few times. The speaker usually goes through the motions of the talk and towards the end makes some comment about "how we should live up to our dedication" and notes that on this occasion there aren't any baptisms but there were xxx thousand last year worldwide.
It always felt a bit weird and pointless but the baptism talk is usually the last talk before lunch at the end of a long morning session so most people don't really pay much attention to it anyway as it tends to be the same talk every time - it always was a good talk to have a good snooze through.
One of the elders in my old congregation had the talk one year & I remember talking to him a month or so before the assembly and he was really worried that there weren't any candidates ( the speaker usually phones the secretaries in the congregations in the circuit to get an idea of numbers a few weeks before the assembly ). He was delighted that at the last minute a really nutty woman decided to get baptised and she was rushed through the questions. She was a total fruitcake who shouted "YES" very loudly at the 2 questions but saved the brothers talk from being a bit of a farce. From what I heard , she stopped attending a few months afterward ( after the love bombing wore off. )
As I have said before - it would make sense for the Society just to have one baptism a year ( at the main convention ) otherwise this will just happen more & more often as less & less people get baptised.