__() not returning original string as fallback when using PHP language file

I think I might have discovered a bug in how the l10n.php localization files that are supported since WP 6.5 are generated (alternatively how they are processed).

When calling __() or other translation functions on a string that does not yet have a translation, the function is supposed to return the original string. Instead, it returns an empty string.

Here is my scenario:

1. I have a custom theme with a text domain. I use wp i18n make-pot to generate a .pot file and also use WP-CLI to update the .po files and generate .mo files.
2. I use wp i18n make-php to also generate the more performant .l10n.php files
3. We are in the process of adding an extra language to our website, so we have created a new .po file for that language which (at the moment) only have a few strings translated

For the untranslated strings, the generated PHP file contains entries such as:
'Log in'=>''

I’ve debugged the issue, and I can see that the problem is that the locate_translation function in the WP_Translation_Controller class contains the following lines:

// Find the translation in all loaded files for this text domain.
foreach ( $this->get_files( $textdomain, $locale ) as $moe ) {
  $translation = $moe->translate( $singular );
  if ( false !== $translation ) {
    return array(
      'entries' => explode( "\0", $translation ),
      'source'  => $moe,
    );
  }

The code expects missing translations to be false, not an empty string. So in this case it will return the empty string as the translation.

This looks like a bug to me.

P.S.: I have already implemented a workaround for my theme. I’ve made a gettext hook that checks for such empty translations and returns the original string instead.