Popup/Tooltip content function: allow return undefined

In this case Popup/Tooltip is not shown.

Usefull 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

@@ -164,7 +164,9 @@ export var DivOverlay = Layer.extend({
 		var node = this._contentNode;
 		var content = (typeof this._content === 'function') ? this._content(this._source || this) : this._content;
 
-		if (typeof content === 'string') {
+		if (!content) {
+			return;
+		} else if (typeof content === 'string') {
 			node.innerHTML = content;
 		} else {
 			while (node.hasChildNodes()) {