Popup/Tooltip content function: allow return undefined

In this case Popup/Tooltip is not shown.

Useful when only some sublayers need popups:
- previously: it was necessary to attach independent popups to each sublayer
- now: it's enough to have common content function attached to parent layer

src/layer/DivOverlay.js

@@ -175,6 +175,10 @@ export var DivOverlay = Layer.extend({
 		this.fire('contentupdate');
 	},
 
+	_hasContentFor: function (layer) {
+		return typeof this._content !== 'function' || this._content(layer || this);
+	},
+
 	_updatePosition: function () {
 		if (!this._map) { return; }
 

src/layer/Popup.js

@@ -434,7 +434,7 @@ Layer.include({
 			latlng = layer.getCenter ? layer.getCenter() : layer.getLatLng();
 		}
 
-		if (this._popup && this._map) {
+		if (this._popup && this._map && this._popup._hasContentFor(layer)) {
 			// set popup source to this layer
 			this._popup._source = layer;
 

src/layer/Tooltip.js

@@ -328,7 +328,7 @@ Layer.include({
 			latlng = layer.getCenter ? layer.getCenter() : layer.getLatLng();
 		}
 
-		if (this._tooltip && this._map) {
+		if (this._tooltip && this._map && this._tooltip._hasContentFor(layer)) {
 
 			// set tooltip source to this layer
 			this._tooltip._source = layer;