DEBUG=true still prints the WOPI token key, the metrics shared secret and the proxy credentials

[DeepDiver1975]

#543 moved the secret-bearing environment variables to the "${VAR:+x}" form so
that DEBUG=true no longer prints their values, but three were missed and still
use the plain [[ -z "${VAR}" ]] test:

• OWNCLOUD_WOPI_TOKEN_KEY (v24.04/overlay/etc/entrypoint.d/85-others.sh)
• OWNCLOUD_METRICS_SHARED_SECRET (v24.04/overlay/etc/entrypoint.d/85-others.sh)
• OWNCLOUD_PROXY_USERPWD (both variants, a user:password pair)

With DEBUG=true the container log therefore contains, for example:

+ [[ -z WopiTokenSecret1111111111111111111111 ]]

Expected: the trace shows [[ -z x ]], as it already does for
OWNCLOUD_DB_PASSWORD and the other covered secrets.