Fix optimized `compare` when first arg is Nil

[brian-carroll]

Fix a "truthiness" bug in the PROD version of _Utils_cmp

This line wants to check if the first argument is a Tuple. In PROD mode it uses if (!x.$) to check for falsiness, since tuples have no $ property in PROD mode.

Unfortunately _List_Nil can also be passed in here, which is represented as { $: 0 }. When that happens, x.$ exists, but since it is falsy it passes the condition and is misidentified as a tuple! The tuple-related code then runs even though the inputs are lists. This can produce incorrect results, such as in the SSCCE below.

This PR fixes it by using hasOwnProperty instead of a truthiness check.

(There are, of course, lots of ways to check for missing properties. Not sure what your preference is. I figured that checking the existence of a property should be more efficient than doing anything with its value. Dunno if that's right or not.)

module Main exposing (main)

import Browser
import Html exposing (Html, text)

main = Browser.sandbox
    { init = ()
    , view = view
    , update = \() () -> ()
    }

view : () -> Html msg
view () =
    if [] < [0] then
        text "Default compile shows this"
    else
        text "--optimize shows this"