ApiEchoNotifications: Don't add wikis with no timestamp
ApiEchoNotifications: Don't add wikis with no timestamp

This should prevent fatals in the sort-by-timestamp code
farther down, and also prevent incorrect output.

Bug: T139112
Change-Id: I6480b2e751594ba3392322f40e950f95ae702104

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo