fix(olHelper): change null layer push to empty ImageLayer
fix(olHelper): change null layer push to empty ImageLayer

The problem is that starting with ol v3.8.2 it is no more supported to
push null onto the layers collection like it was before:

```javascript
layers.push(null);
```

milafrerichs/angular-openlayers-directive,chambleton/angular-openlayers-directive,birdlifedk/angular-openlayers-directive,mikostn/angular-openlayers-directive,gaf-ag/angular-openlayers-directive,TeChn4K/angular-openlayers-directive,StevenHeinrich/angular-openlayers-directive,ageblade/angular-openlayers-directive,mikostn/angular-openlayers-directive,claycephas/angular-openlayers-directive,tombatossals/angular-openlayers-directive,polesapart/angular-openlayers-directive