Fix empty string handling in push notifications

Fixes https://github.com/vector-im/riot-web/issues/11460

Empty strings are falsey, and the state key match for a tombstone event is an empty string. Ergo, nothing happens because all the conditions fail.

src/utils.js

@@ -714,3 +714,7 @@ module.exports.ensureNoTrailingSlash = function(url) {
 module.exports.sleep = (ms, value) => new Promise((resolve => {
     setTimeout(resolve, ms, value);
 }));
+
+module.exports.isNullOrUndefined = function(val) {
+    return val === null || val === undefined;
+};