Make Antlers value resolution "lazy", and make pluck operator work with nulls.

[JohnathonKoster]

This PR closes #7148

The first fix just checks to make sure that the pluck operator doesn't call the internal Arr::pluck when the incoming value is null.

The second fix makes the evaluation of values "lazy", so that the following only evaluates the tag when value is truthy:

{{ undefined_variable ?= {mytag} }}

<?php

namespace App\Tags;

use Statamic\Tags\Tags;

class Mytag extends Tags
{
    public function index()
    {
        dump('tag is executed');

        return 'test';
    }
}

This lazy evaluation also extends to logic groups, so the following will no longer prematurely set the test variable:

{{ var_exists ?= (test = "testing") }}

{{ test }}

Test cases have been provided for both items 👍

[what-the-diff]

• LogicGroup is now evaluated as a whole, instead of evaluating the right side first.
• Added test for pluck on variable that doesn't exist (returns empty string).