Fix tooltip focus listener if getElement is no function

[Falke-Design]

If a LayerGroup contains other layers without the getElement() function (for example another layergroup), it will throw an error, because it loops over all layers in the LayerGroup:

Leaflet/src/layer/Tooltip.js

Lines 388 to 395 in 9b9c712

	_addFocusListeners() {
	if (this.getElement) {
	this._addFocusListenersOnLayer(this);
	} else if (this.eachLayer) {
	this.eachLayer(this._addFocusListenersOnLayer, this);
	}
	},

Another solution (and I think the cleaner one) would be to loop over all layers and calling _addFocusListeners instead of _addFocusListenersOnLayer. But I'm a little bit scared about endless recursion.

 _addFocusListeners() { 
 	if (this.getElement) { 
 		this._addFocusListenersOnLayer(this); 
 	} else if (this.eachLayer) { 
 		this.eachLayer(this._addFocusListeners, this);  // <-- into _addFocusListeners instead of _addFocusListenersOnLayer
 	} 
 }, 

This fix needs to be cherry-picked into v1

[Falke-Design]

@jonkoops @mourner @IvanSanchez please review, I would like to release a new 1.9 version soon.