fix: Disable logging unknown data-type in console

[manzt]

Description

For whatever reason, the evtJson is often a boolean here:

    if (!(datatype in DEFAULT_TRACKS_FOR_DATATYPE) && !evtJson.defaultTracks) {
      console.warn('unknown data type:', evtJson.higlassTrack);
      return undefined;
    }

and therefore evtJson.defaultTracks is undefined, leading to the console erroring:

unknown data type: undefined

consistently.

What was changed in this pull request?

I've seen this log for ages in both Gosling.js and HiGlass but never had the time to
look very deeply into it.

Why is it necessary?

Fixes #___

Checklist

• Set proper GitHub labels (e.g. v1.6+, ignore if you don't know)
• Unit tests added or updated
• Documentation added or updated
• Example(s) added or updated
• Update schema.json if there are changes to the viewconf JSON structure format
• Screenshot for visual changes (e.g. new tracks or UI changes)
• Updated CHANGELOG.md