Can set the opacity of a null icon

[charlie67]

Steps to reproduce
Steps to reproduce the behavior:
Difficult to reproduce, and our code can't be shared online.

Expected behavior
Not setting the opacity when the icon is null.

Current behavior
This occurs when using ember-leaflet-marker-cluster alongside leaflet and the way ember-leaflet-marker-cluster is being used ithe opacity of a null icon tries to be set which results in this exception:
Uncaught TypeError: Cannot read property 'style' of null at setOpacity (leaflet-src.js:2368) at NewClass._updateOpacity (leaflet-src.js:7694) at NewClass.setOpacity (leaflet-src.js:7684) at NewClass.clusterShow (leaflet.markercluster.js:2) at leaflet.markercluster.js:2 at NewClass._recursively (leaflet.markercluster.js:2) at NewClass._recursivelyRemoveChildrenFromMap (leaflet.markercluster.js:2) at NewClass._moveEnd (leaflet.markercluster.js:2) at NewClass._mergeSplitClusters (leaflet.markercluster.js:2) at NewClass._zoomEnd (leaflet.markercluster.js:2)

Environment

• Leaflet version: 1.4.0
• Browser (with version): Firefox 66.0.2, Chrome 73.0.3683.103
• OS/Platform (with version): Ubuntu 18.04.1

Additional context
Managed to fix this locally by changing the _updateOpacity method in src/layer/marker/Marker.js file and changing it to check if the icon is set before updating the opacity change it from:
DomUtil.setOpacity(this._icon, opacity)
to
if (this._icon) { DomUtil.setOpacity(this._icon, opacity); }

This seems like a very small change so shouldn't break anything and it would help us out loads.
I'll add in a pull request with the change I mentioned

Minimal example reproducing the issue
As I mentioned it's difficult to reproduce and we can't share our code online. If the code needs to be reproduced call the _removeIcon method, then call the _updateOpacity method.

[mourner]

Fixed by #6610