fix: return null for avg() of empty array

[spokodev]

Bug

avg() over an empty array returns NaN instead of null:

jmespath.search({e: []}, 'avg(e)'); // => NaN, expected null

The cause is _functionAvg computing sum / inputArray.length, which is 0 / 0 = NaN when the array is empty.

Spec

The JMESPath specification for avg() states: "An empty array will produce a return value of null." (https://jmespath.org/specification.html#avg)

The official compliance suite already has the case avg(empty_list) -> null in functions.json. The copy vendored in this repo's test/compliance/functions.json predated that case, so the gap went unnoticed.

Fix

Guard the empty case at the top of _functionAvg:

_functionAvg: function(resolvedArgs) {
    var sum = 0;
    var inputArray = resolvedArgs[0];
    if (inputArray.length === 0) {
        return null;
    }
    for (var i = 0; i < inputArray.length; i++) {
        sum += inputArray[i];
    }
    return sum / inputArray.length;
},

Also adds the missing compliance case avg(empty_list) -> null to test/compliance/functions.json (the empty_list input already exists in the suite's given block).

The added test fails before the fix (NaN should loosely deep-equal null) and passes after. Full suite green: 889 passing.