Fix empty string usage if translations are missing

[Elblinator]

Describe the bug
If the translations from a string are missing and the strings are delivered by the backend then empty strings are used to translate the string

How to Reproduce
Two examples:

1. Set your browser default language to Spanish

2. In the es.po file remove the translated string from "You are receiving this email because you have requested a new password for your OpenSlides account..." and from "Email subject"

3. Send yourself a reset email the subject line and the content are empty

4. from the es.po remove the translation from "Delegates"

5. Create a new meeting in spanish

6. The following Error is thrown Error: data.name must be longer than or equal to 1 characters

Expected behavior
If the translations in the translation file are missing (aka. are empty strings) then the english strings should be used