Handle logging of "invalid quota <> for LDAP user" differently

As described in the forums and another (closed) issue here, the user_ldap app spams the owncloud.log if no quota is set for a certain user.
The log line looks as follows:

{"reqId":"mdn5o8QncCyPidvVjTMn","level":3,"time":"2018-08-21T09:30:03+02:00","remoteAddr":""
,"user":"--","app":"user_ldap","method":"--","url":"--",
"message":"Invalid quota <> for LDAP user "}

I think this should be handled differently. To be precise I wouldn't treat "no quota" as an invalid one. From what I've experienced in customer projects with owncloud and nextcloud, it's relatively common not to set a quota at all (depends on the project and users of course). This is certainly not a best practice but the log file shouldn't be spammed in this case either in my opinion.

Feel free to suggest different approaches as well.

Start by locating the LDAP quota validation and logging path in the user_ldap app, then reproduce the case with an LDAP user whose quota is unset. Done means an unset quota no longer spams owncloud.log while genuinely invalid quota values continue to be handled appropriately.