Prevent `TypeError: matches is undefined` in Sync.
Prevent `TypeError: matches is undefined` in Sync.

I.e. from an old Sync record which did not specify user matches.  Default to nothing.

Fixes #2188

Ventero/greasemonkey,Martii/greasemonkey,Martii/greasemonkey,the8472/greasemonkey,the8472/greasemonkey,Ventero/greasemonkey