[FIX] web: translation dialog assumes translation value for en_US

Steps to reproduce:
- Create a database with german as main language
- Load the en_US language (keep user preference as german)
- Activate debug mode > find the email template named "Purchase Order: Send PO"
- Edit the mail template > click the "DE" button to show thet translation dialog

Previous behavior:
the en_US translation is empty, it is confusing because the user assumes
that english translation have not been loaded.
that translation is actually fine when you switch to english

Current behavior:
en_US translations' empty value are assumed to be the same as the source

opw-2276673

closes odoo/odoo#53787

X-original-commit: 52975811071af3dbfe31d93b19f8a0f3a1b6b694
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Signed-off-by: mightyjol <jhk-odoo@users.noreply.github.com>
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/web/static/src/js/widgets/translation_dialog.js

@@ -64,6 +64,9 @@ odoo.define('web.TranslationDialog', function (require) {
             ]).then(() => {
                 this.data = this.translations.map((term) => {
                     let relatedLanguage = this.languages.find((language) => language[0] === term.lang);
+                    if (!term.value && !this.showSrc) {
+                        term.value = term.src;
+                    }
                     return {
                         id: term.id,
                         lang: term.lang,