fix: check whether _map is assigned to avoid errors on often redraw from scratch

[utilmind]

This commit fixes the issue with attempt to draw points on uninitialized map.

Reproduce on https://wio2.utilmind.com/demo/

1. Go to the page by link above and open browser console to see errors.
2. Wait until map is loaded.
3. Turn on "Weather" layer, which is based on glify.points().
   
4. Drag slider to switch weather frames too fast and too often.
   

This fix only inserts validation whether _map is not empty to line # 168. (And the rest of changes are just shift due to formatting).