MB-7510 Computed stats of 'null' arguments are also 'null'.

It's possible to have a list of samples where the first sample
contains some stats that are missing from preceding stats. Those
missing stats in the older samples will be replaced by 'null'
values. But computed stats calculated from such samples will fail
because they expect normal values. So we'll just assume that computed
stat of 'null' argument (or arguments) is also 'null'.

This can happen if, for some reason, we were unable to grab couch
stats for some time. Or just during upgrade.

Change-Id: I6a842dc8cfe68d50c3614d0ab36a98a4a33693d6
Reviewed-on: http://review.couchbase.org/23961
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>