lib.gvariant: remove throwIf usage

throwIf sends our error message through a function call, even if the
error condition doesn't trigger. This requires a lot of thunk allocation
that can be easily avoided.

lib/gvariant.nix

@@ -191,7 +191,9 @@ rec {
   mkArray =
     elems:
     let
-      vs = map mkValue (lib.throwIf (elems == [ ]) "Please create empty array with mkEmptyArray." elems);
+      vs = map mkValue (
+        if elems == [ ] then throw "Please create empty array with mkEmptyArray." else elems
+      );
       elemType = lib.throwIfNot (lib.all (t: (head vs).type == t) (
         map (v: v.type) vs
       )) "Elements in a list should have same type." (head vs).type;