Empty environment variable TRYTOND_DATABASE_URI treated as set

Typical behaviour in the shell is to treat existing environment variables containing nothing the same as a if the env variable is not defined.

trytond.config does not behave like this:

$ env -u DB_NAME -u TRYTOND_CONFIG -u TRYTOND_DATABASE_URI python -m unittest discover -vvvvv -s modules/party/tests/
…
Ran 51 tests in 16.400s
OK
$ TRYTOND_DATABASE_URI= env -u TRYTOND_CONFIG -u DB_NAME python -m unittest discover -vvvvv -s modules/party/tests/
…
ModuleNotFoundError: No module named 'trytond.backend.'
…

trytond-6.8.0

Expected behaviour: both commands work the same way

This is caused by https://foss.heptapod.net/tryton/tryton/-/blob/trytond-6.8.2/trytond/trytond/config.py#L60-61