dvr: Fix autorec if it has a category but event has no category. (#4777).

dvr: Fix autorec if it has a category but event has no category. (#4777).

Although we use a drop-down list for autorec categories, if the
user has no categories enabled (such as OTA) and creates an autorec
with a category then it would match all events.

Now we fix it that events without a category can never match an
autorec with categories.

Issue: #4777.