The service does not start if MYSQL_ROOT_PASSWORD is not especified.

**Situation**:

I'm using an existing docker container for the database. I already created a table and a user for Wallabag. As there is no need to create a user or password on the database for Wallabag, I skipped the variable `MYSQL_ROOT_PASSWORD`.

**Result**:

Once deployed using `docker-compose`, the app never started. I had to remove both the user and the table for getting Wallabag working.

**Expected** result:

As the table and the user already existed on a running database container, I expect that Wallabag checks if the database and/or the user exist, populating the database or creating the user accordingly.