Two fixes/improvements to `gitea_json_issue_events_filter()`:
- The `username` parameter would be optional, but not setting it would
make the function do nothing. Ignore it if not set.
- Not passing `event_type` (and not passing `labels`) would make the
function do nothing. It wasn't optional strictly speaking but the
default was an empty set, which is more or less the same. Make it
properly optional so not specifying an event-type will return events
of any type.
5de40adc9e