"in" expression always returns false if needle argument is null

[lbutler]

mapbox-gl-js version: 1.9.0

Steps to Trigger Behavior

1. Use an in expression that has the needle as null with the haystack as an array containing null - e.g. `['in', null, ["literal", [1,2,3,null,5] ]]
2. The expression will always return false

Link to Demonstration

https://jsbin.com/qepihirize/edit?js,output

Each circle is a needle of a different type, they should all be black as they are within the haystack.

Expected Behavior

Be able to find null within an array

Actual Behavior

in will always return false

---

When implementing the index-of expression (#9443) I noticed that in will always return false if the needle is null even if the haystack is an array of values that contains a null.

The line that is causing this issue is below, it appears that it was changed in #9295 to allow false but still filters out null and undefined and shortcuts to return false.

mapbox-gl-js/src/style-spec/expression/definitions/in.js

Line 65 in 0e04a3a

if (needle == null || !haystack) return false;

I'd expect index-of and in to work the same way, just so we don't get conflicting answers for the below edge case:

['in', null, ["literal", [1,2,3,null,5] ]]                     // false
["==", ['index-of', null, ["literal", [1,2,3,null,5] ]], -1]   // true

I can fix this as part of my PR #9450 but just wanted to confirm that this wasn't another reason why we have this behaviour.

[lbutler]

@ryanhamley provided some extra information about this bug as part of the review of the slice/index-of expression PR #9450 (comment)