(PUP-2817) Fix zone properties so that nil is not passed through

Before this patch, when insync? was called on the zone properties ip,
dataset and inherit with value nil (happens the first time when the properties
are not present), true was returned, which resulted in them not being set
until the values were actually queried, which happened after the installation
of zone. Unfortunately, the inherit property can only be set before
installation.

This patch removes the spurious check for nil, which always returned true,
and instead handles it similar to when :absent is used.