At the risk of stating the obvious, heads aren't made of liquid
Even Jenova has a physical head.
*Some* liquid leaked from the container after Rufus shot it, but nothing suggests its entire contents was lost. Later, after Kadaj severs the top of the container over Cloud's sword, some more of that green fluid sprays out, but he's left clutching an object of some kind, just as Xurts said.
There must've been some kind of significant amount of Jenova's head left - after all, the Turks found it at the Northern Cave. They'd hardly have known they'd found Jenova's remains if there was just a pool of green goo; there were plenty of those in the crater to begin with.