Fix log file check

[ArnoStiefvater]

What:

Adds missing handling of syslog logging and empty file string to #447 .

Why:

Else not able to init openvas with some logging configs.

How:

add the following to the openvas_log.conf

[sd   main]
prepend=%t %s %p
separator=:
prepend_time_format=%Y-%m-%d %Hh%M.%S %Z
file=syslog
syslog_facility=daemon
syslog_indent=openvas
level=127

Without the patch openvas -s will fail if openvas is started as other user (e.g. sudo -u testuser openvas -s). Same if file= is put into the config. With applied PR this is fixed.

Checklist:

• Tests
• CHANGELOG Entry