check whether notif level is undefined, because 0 is falsey (#651)

* check whether notif level is undefined, because `0` is falsey and it failed

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* improve number check for all m.room.power_levels related stuffs

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/utils.js

@@ -652,3 +652,13 @@ module.exports.inherits = function(ctor, superCtor) {
         },
     });
 };
+
+/**
+ * Returns whether the given value is a finite number without type-coercion
+ *
+ * @param {*} value the value to test
+ * @return {boolean} whether or not value is a finite number without type-coercion
+ */
+module.exports.isNumber = function(value) {
+    return typeof value === 'number' && isFinite(value);
+};