Make dictionary work with event definitions without category

Adds support for event definitions internal_events: true and no category. See #11 (closed) for more context.

Steps to verify:

1. Seed the data: SKIP_INTRODUCED_BY_URL_AUTOLOADING=1 bin/seed
2. Edit event_data.json - add internal_event: true to one event.
3. Build and run the metric dictionary:

   bin/build
   yarn --cwd ./web
   yarn --cwd ./web dev

4. Open the Snowplow events in the Metric Dictionary in your browser: http://localhost:9080/snowplow
5. Search for the event you modified in step 2) and verify that the category shows as InternalEventsTracking

Closes #11 (closed)