fix(#0089): fall back to generic value when custom translation is null

[SigifredoChaconB]

Description

Fixes translation values loading in the edit translation modal so that keys with null values
in custom fall back to the generic value.

Changes include:

• Updated translationValues loading in DisplayTranslationsEditComponent to use custom[key] || generic[key]
  instead of spread operator, matching the behavior of the original app
• This fixes the case where translation keys saved with null in custom by the original AngularJS admin
  tool caused all language fields to appear empty when opening the edit modal in the new admin tool
• Added unit test to cover the null custom value fallback to generic

Code review checklist

• Readable: Concise, well named, follows the style guide, documented if necessary.
• Tested: Unit and/or e2e where appropriate
• Internationalised: All user facing text

License

The software is provided under AGPL-3.0. Contributions to this project are accepted under the same license.