fix(Dunning): logic for fetching text

[barredterra]

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)