My personal feelings differ from Sand and many other collectors. I would empty the contents on the off chance they get knocked over and broken. What they contain may either smell bad by now, stain whatever surface they possibly break on, and the contents add zero value to the bottles collectability IMO. If you really want them to be displayed with contents I'd suggest washing them out, adding water and a few drops of food coloring so if they do happen to get broken you aren't dealing with what I mentioned above... Just my two cents, which if you add $5 to, will get you a Venti frappucinno at Starbucks.
99.9 percent of the time I just like looking at the glass. But in this case, if you were to take away the lids, and then the labels, and then the contents, there'd be almost nothing interesting left to look at, because these are just plain clear bottles underneath it all.
So yeah, if I did collect this sort of thing, in this case I guess I might keep the contents! If it were some beautiful colourful pontiled bottle with some lumpy brown sludge sloshing around inside, I'd probably clean it out...