nixos/nextcloud: fix inclusion of trusted_domains in override config

Regression I caused with 3944aa05, sorry
for this! The Nextcloud installer broke back then because
`trusted_domains` was an empty value by default (a.k.a an empty array)
which seemed to break the config merger of Nextcloud as Nextcloud
doesn't do recursive merging and now no domain was trusted because of
that, hence Nextcloud was unreachable for the `curl` call.