fix exception in mouse event handler with empty CanvasMarkerLayer

`this._markers` is undefined in `_searchPoints`, called from `_onMouseMove`
Could be fixed in some another place:
e.g. check `_markers` in `_searchPoints`
 ...or initialize (empty) `_markers` on CanvasMarkerLayer init
 ...or do not attach listeners until we have markers
 ...or check markers existence in the beginning of listener

src/plugin/leaflet.canvas-markers.js

@@ -298,7 +298,7 @@ function layerFactory(L) {
             }
         },
         _onMouseMove: function (e) {
-            if (!this._map || this._map.dragging.moving() || this._map._animatingZoom) { return; }
+            if (!this._map || !this._markers || this._map.dragging.moving() || this._map._animatingZoom) { return; }
 
             var point = e.containerPoint;
             this._handleMouseHover(e, point);