The problem is best described by someone else's question:
I'm trying to use the generic list environment inside a longtable. Unfortunately I get unexpected space above and below the list environment. If I use the same list between normal paragraphs, this extra space is not there.
Probably the "res" package uses some tabular environment. It seems that unneeded vertical space before and after a list in a cell is a known problem without a good solution. In your case, instead of hardcoding sizes, I'd try something like this (probably, with \null):
I don't know why \topset doesn't work. I suspect that that table cell starts in the horizontal mode, and when a list adds a break penalty, TeX creates an empty paragraph due to switching from the horizontal mode to the vertical mode.
By the way, for my longtables I use the following commands:
Probably it's only a coincide that these commands work for me.