Fix Nunjucks default() values when null, false or "" is provided

[colinrotherham]

This PR ensures Nunjucks default() handles all possible "falsey" values

It closes #4305 which shows that undefined values fall back to defaults() but not null, false, "" etc

Nunjucks default example

For example, passing redirectUrl: null into Exit this page renders a default submit button 😮

{{ govukExitThisPage({
  redirectUrl: null
}) }}

The fallback href value isn't used:

<button type="submit">…</button>

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-4.7.0.min.css	120.66 KiB
dist/govuk-frontend-4.7.0.min.js	51.56 KiB
dist/govuk-frontend-ie8-4.7.0.min.css	81.59 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	74.96 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	70.36 KiB
packages/govuk-frontend/dist/govuk/all.mjs	3.8 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	113.99 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	36.32 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.3 KiB

Modules

File	Size
all.mjs	66.68 KiB
components/accordion/accordion.mjs	20.38 KiB
components/button/button.mjs	4.4 KiB
components/character-count/character-count.mjs	20.57 KiB
components/checkboxes/checkboxes.mjs	5.48 KiB
components/error-summary/error-summary.mjs	5.69 KiB
components/exit-this-page/exit-this-page.mjs	15.66 KiB
components/header/header.mjs	3.42 KiB
components/notification-banner/notification-banner.mjs	4.24 KiB
components/radios/radios.mjs	4.47 KiB
components/skip-link/skip-link.mjs	3.55 KiB
components/tabs/tabs.mjs	9.16 KiB

View stats and visualisations on the review app

---

Action run for 61b64d4

[domoscargin]

When will this trigger? My brain's spinning in circles...

[colinrotherham]

It's a weird case where govukSelect() can have a selected value but the item itself is unselected 🤯

So we want to keep the falsey value as long as it's actually false and not null or ""

govuk-frontend/packages/govuk-frontend/src/govuk/components/select/select.yaml

Lines 350 to 358 in 72c543a

	items:
	- value: red
	text: Red
	- value: green
	text: Green
	selected: false
	- value: blue
	text: Blue
	value: green

It's covered by this test case:

govuk-frontend/packages/govuk-frontend/src/govuk/components/select/template.test.js

Lines 101 to 106 in 72c543a

	it('allows item.selected to override value', () => {
	const $ = render('select', examples['item selected overrides value'])
	const $selectedItem = $('option[value="green"]')
	expect($selectedItem.attr('selected')).toBeUndefined()
	})

[domoscargin]

Looks good, I just need an explainer on the item.selected stuff because it's still too early for my brain.

[domoscargin]

I was going to do a foot emoji and a bug emoji for squashing the bug, but let's go with releasing the bug into the garden instead.

🐛 🏞️

[colinrotherham]

Thanks @domoscargin just added a CHANGELOG entry too