SitemapService: check for `uri` IS NOT NULL

[7ochem]

Changes proposed in this pull request:

• Add a check for uri IS NOT NULL in query when looking up for which sites the element/item is enabled

---

I have these records in my database:

+---------+-----------+--------+--------------+--------------+---------+---------------------+---------------------+--------------------------------------+
| id      | elementId | siteId | slug         | uri          | enabled | dateCreated         | dateUpdated         | uid                                  |
+---------+-----------+--------+--------------+--------------+---------+---------------------+---------------------+--------------------------------------+
| 4425315 |   2370031 |      1 | met-dank-aan | met-dank-aan |       1 | 2022-08-23 08:48:18 | 2022-08-23 08:48:55 | cb78860e-3edd-4053-8b50-c3b38bf84904 |
| 4425316 |   2370031 |      2 | met-dank-aan | NULL         |       1 | 2022-08-23 08:48:19 | 2022-08-23 08:49:04 | 206f1529-5cd2-4d6f-b1eb-8a90dbde22ff |
+---------+-----------+--------+--------------+--------------+---------+---------------------+---------------------+--------------------------------------+

which caused the error:

2024-04-11 13:15:49 [web.ERROR] [TypeError] TypeError: craft\helpers\UrlHelper::siteUrl(): Argument #1 ($path) must be of type string, null given, called in vendor/ether/seo/src/services/SitemapService.php on line 398 and defined in vendor/craftcms/cms/src/helpers/UrlHelper.php:306 Stack trace: #0 vendor/ether/seo/src/services/SitemapService.php(398): craft\helpers\UrlHelper::siteUrl(NULL, NULL, NULL, 2) #1 vendor/ether/seo/src/controllers/sitemap/XmlController.php(25): ether\seo\services\SitemapService->core(Array)