Before

(a). If the language parameter is provided, return the text for this language. If this doesn't exist, return None.
(b.) If the language parameter is not provided, return the text marked as Is Default Language. If this doesn't exist, return None.

After

1. If the language parameter is provided, return the text for this language.
2. If doc.language exists, return the text for this language.
3. If a text marked as Is Default Language exists, return this one.
4. Return an empty dict (-> uniform return type)

How to test

1. Create a Dunning Type
   • add texts for different languages, enable Is Default Language for one of them
2. Create overdue Sales Invoices for customers with different languages
3. Click on "Create" -> "Dunning" in each Sales Invoice

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   This is an automatic backport of pull request fix(Dunning): logic for fetching text #43160 done by Mergify.