I'm coming to this a bit late, and I'm not certain (I just started modding Vintage Story, for myself in single player, a couple days ago), but I think what you would want to do is:
Make sure that your "drops the leaf block when a knife is used" entry in the drops list is the first one.
Add
"LastDrop": true,
into the "drops a leaf block when a knife is used" attributes.
My understanding from reading the docs is that when you set "LastDrop" to true, no further drops will occur if the one flagged with "LastDrop" happens. So, if you make that the first one that is checked, and then it turns out the player is using a knife, it should cause the leaf block to drop and then it should stop checking to see if anything else should drop because the leaf block was flagged as the last drop. However, if the player isn't using a knife and the leaf block doesn't drop, it should move on and check all the remaining drops like normal.
