Improve some null types to prepare for strictNullChecks

src/ol/dom.js

@@ -83,7 +83,7 @@ export function replaceNode(newNode, oldNode) {
 
 /**
  * @param {Node} node The node to remove.
- * @return {Node} The node that was removed or null.
+ * @return {Node|null} The node that was removed or null.
  */
 export function removeNode(node) {
   return node && node.parentNode ? node.parentNode.removeChild(node) : null;