Correct "is this palette empty?" function

This notably caused decantation to delete non-empty palettes, which crashes

src/gfx/pal_packing.cpp

@@ -157,7 +157,7 @@ public:
 	void clear() { _assigned.clear(); }
 
 	bool empty() const {
-		return std::find_if_not(
+		return std::find_if(
 		           _assigned.begin(), _assigned.end(),
 		           [](std::optional<ProtoPalAttrs> const &slot) { return slot.has_value(); })
 		       == _assigned.end();