I assumed without questioning it that you shuffle with one card but not with zero
That's certainly one of the 3 possible interpretations; though according to
this poll the least-common one. Granted, the poll is not unbiased and happened after much discussion around this had already taken place.
Of course, the power level of Tax Collector varies greatly depending on which interpretation you use. If 0 cards counts as shuffling, then man is it insanely overpowered. Every single card-drawing card gets a +1 Coffer tacked on if you've drawn your deck.
Also a weird thing... if 0 cards counts as shuffling, then Smithy works very differently under first edition rules compared to second edition rules. In second edition rules, when you play Smithy with an empty deck, you would first shuffle your discard pile, and then draw 3 cards (failing to draw any), for 1 shuffle. In original rules, you would "shuffle" each time you go to draw 1 of the cards for Smithy, getting 3 shuffles in total.
Now I'm conflicted... although I think lots of people may assume that "when you shuffle" triggers when you shuffle an empty pile, I don't feel like anyone would have ever assumed that playing a Smithy while your deck is empty would count as shuffling 3 times.
*Edit* And I think I know why. In MTG, you only ever shuffle your library when an effect specifically instructs you to "shuffle your library". Because an effect tells you to do that, people don't think it's all that weird when the rules clarify that even if you have 0 cards in your library, it still counts as having shuffled your library when you're instructed to do so. But in Dominion, shuffling happens automatically whenever your draw pile is empty and you need cards. If your discard pile is also empty, people don't think in terms of "now my draw pile is empty, so I will attempt to shuffle my discard pile". Rather they simply think "oh, because my discard pile is empty, the normal rule that says to make a new draw pile doesn't apply in this situation". So it's not so much a question of "does it count as shuffling if it is only 0 cards" as much as it is "do you still even bother making a new draw pile if your discard pile is also empty".
*Edit again* I think the simple fix is to say that yes, it still counts as shuffling if you only have 0 cards to shuffle, BUT the rule about what to do when your draw pile is empty / doesn't have enough cards to do what you need is modified to say "if your discard pile has any cards in it, shuffle your discard pile and put it on the bottom of your draw pile".