@returns {string|null} or @returns{?string} do not allow "return null"

When my JSDoc contains `* @returns {string|null}` or `* @returns {?string}`, a `return null;` statement in the following function is not allowed, giving me:

```
Wrong returns value at lib/myfile.js :
    57 |MyClass.prototype.getIp = function (version) {
    58 | if (!this.addresses) {
    59 |  return null;
-----------------^
```
