Do not call html() on an empty carrier extra content collection

[ShaiMagal]

Questions	Answers
Branch?	2.2.x
Description?	updatedDeliveryForm calls .html() on the result of .next('.carrier-extra-content'). When that selector matches nothing, jQuery returns an empty collection, .html() returns undefined and .trim() throws on every carrier change. This adds the missing length check.
Type?	bug fix
Category?	FO
BC breaks?	no
Deprecations?	no
How to test?	See below.
Fixed issue or discussion?	#214
Sponsor company	OpenServis.cz

Why

Nothing guarantees that .carrier-extra-content stays the immediate next sibling of .delivery-option. Any element inserted in between makes .next() return an empty collection, which is a common situation once a pickup point module places its selector next to the carrier row.

The exception is also hard to diagnose, because jQuery 3 converts it into a rejection and the .fail() branch in themes/_core/js/checkout-delivery.js then calls prestashop.trigger(...), a method the emitter does not have. The console only shows r(...).trigger is not a function, and the real cause stays hidden. That second problem lives in the main repository and is not addressed here.

The guard being fixed was introduced in 2.1.x by acd52d3 (#157) to solve PrestaShop/PrestaShop#37194, and this change keeps that behaviour: the extra content is still only slid down when it actually has content.

The develop branch is not affected, it calls slideDown() directly and jQuery ignores an empty set there.

How to test

1. PrestaShop 8.2.1 or later with the Classic theme, at least two carriers in the checkout, one of them rendering a .carrier-extra-content block.
2. On the shipping step, run $('.delivery-option').first().after('<div></div>'); in the console to simulate a module inserting an element between the carrier row and its extra content.
3. Select another carrier.
4. Before this change the console shows a TypeError. After it, the carrier switch is silent and the extra content of the selected carrier still expands when it has content.