trivial-builders: fix evaluation when entries is empty (#490790)

        # This is the best we can do since the other attrs are either defined here, or curried values that

        # we cannot extract a position from

        pos =

          if lib.isAttrs entries then

          if (lib.isAttrs entries) && (entries != { }) then

            builtins.unsafeGetAttrPos (builtins.head (builtins.attrNames entries)) entries

          else

            null;