Beats: Allow to add condition that matches all events in autodiscover templates

[jsoriano]

Currently there is no explicit way to define a condition that matches all events in autodiscover templates, we could make that the empty condition matches all, or and some kind of condition: true.

[ruflin]

What about condition: *?

[exekias]

I would say no condition field should match everything, the same way processors without a when clause are always applied

[jsoriano]

Fixed by #9029