This occurred in a two player game last week.

Entering the (final) buildings round there was one worker left so the 
game was guaranteed to end next worker round. Towards the end of the 
buildings round I used the Observatory to pick up that worker card. I 
mistakenly (I believe) thought it would now be game end at the end of 
the current buildings round (and I would win because my opponent had 3 
scoring aristocrats in hand)

 From reading the rules I think they imply the game end does not happen 
until the next worker round despite there being no workers to draw.
[Sadly this meant my opponent got to buy his aristos, scored a few 
points for them in the aristo round and with his extra 17 points for now 
leading me 9 distinct nobles to 7 overhaul me   :-(  ]

I assume that we played correctly by continuing to the next worker round 
despite there being zero workers rather than ending on the building 
round where the last worker was drawn.

Brian