Fix rendering of Back link's `href` and `text` for falsy values

[romaricpascal]

Reverts to the behaviour of 5.4.0, which ensured consistency across falsy values, and importantly that a 'Back' text was displayed in case of empty strings (and a '#' value for the href attribute, but there's less difference with a '').

Adds tests to ensure we keep an eye on the behaviour in future template changes, or at least make sure that any deviation is intentional (and tested).

Instead of reverting the commit, I used the boolean option from Nunjucks' default filter to keep the tidy up of the templates introduced in https://github.com/alphagov/govuk-frontend/commit/fc704d1efc0fc15a69ff84eb18050dd7ba7ce4b3.

Fixes #5189

[github-actions[bot]]

:clipboard: Stats

File sizes

File	Size
dist/govuk-frontend-development.min.css	112.61 KiB
dist/govuk-frontend-development.min.js	41.83 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	87.33 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	82.03 KiB
packages/govuk-frontend/dist/govuk/all.mjs	981 B
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	112.6 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	41.82 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.55 KiB
packages/govuk-frontend/dist/govuk/init.mjs	4.86 KiB

Modules

File	Size (bundled)	Size (minified)
all.mjs	79.16 KiB	39.79 KiB
accordion.mjs	23.5 KiB	12.39 KiB
button.mjs	5.98 KiB	2.69 KiB
character-count.mjs	22.4 KiB	9.92 KiB
checkboxes.mjs	5.83 KiB	2.83 KiB
error-summary.mjs	7.89 KiB	3.46 KiB
exit-this-page.mjs	17.1 KiB	9.26 KiB
header.mjs	4.46 KiB	2.6 KiB
notification-banner.mjs	6.26 KiB	2.62 KiB
password-input.mjs	15.15 KiB	7.25 KiB
radios.mjs	4.83 KiB	2.38 KiB
skip-link.mjs	4.39 KiB	2.18 KiB
tabs.mjs	10.05 KiB	6.06 KiB

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Action run for 6d99967f4dda9938727cc01efee447859b97bc73