DB: Force ttl column to accept NULL values

Specify that ```oc_mozilla_sync_wbo.ttl``` is explicitly allowed to
include NULL values.

See #59
Revert "Check if modified and sortindex are set"

This reverts commit 901d02a3a28a5d6a4255eeac519ef43daf981314.
Check against !isset() instead of is_null()

See issue #66.