[eDrive] Prevent NPE & refactoring

Description

In order to look for potential cause of ANR, I found some case associated with DavClientProvider that could lead to NPE. Also did some code cleaning while looking over class for potential exception

This is not the whole solution to the issue linked, but a part of it.

Screenshots

Technical details

Tests

Issues

https://gitlab.e.foundation/e/os/backlog/-/issues/1398

Goal #3 of July's end iteration

10 commandments of code reviews

👪 ❤ https://gitlab.e.foundation/internal/wiki/-/wikis/mobile-team/guidelines/Code-review