.. instead of `['']`.

The final result in the `env` file will be the same (`POSTMOOGLE_ADMINS=`)
in both cases, but it's better to avoid confusion.

* postmoogle: add admins

* postmoogle: update jija2 defaults

* postmoogle: fix typo

Before this, it was trying to pull the `latest` git branch.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2043