Fix issue when DNS zone import fails silently with OK message if $TTL is...

Fix issue when DNS zone import fails silently with OK message if $TTL is missing from the input file because TTL can not be null when saving SOA to database.
Solution: if TTL is null then simply set it to 1 day - 86400 seconds (as per RFC 1912 recommendations).
I encountered few services that export a dns zone without global ttl set (ex: GoDaddy).