Fix build error

src/lib/Operators/Comparison.vala

@@ -12,7 +12,7 @@ namespace Invercargill {
             return (a, b) => (int)(((uint64)a) - ((uint64)b));
         }
         else if(type == typeof(double)) {
-            return (a, b) => (int)(((double)a) - ((double)b));
+            return (a, b) => (int)(((double?)a) - ((double?)b));
         }
         else if(type.is_a(typeof(Comparable))) {
             return (a, b) => ((Comparable<T>)a).compare(b);