feat(gatsby-plugin-sitemap): add siteUrl validation (#16329)

* empty sitemap Url validation

* creating awareness for siteUrl validation

* site url validation

* url validation fix

* Update creating-a-sitemap.md

* Update internals.js

docs/docs/creating-a-sitemap.md

@@ -26,6 +26,8 @@ module.exports = {
 }
 ```
 
+**Note:** The siteUrl property must be defined and not left empty.
+
 Next run a build (`npm run build`) since the sitemap generation will only happen for production builds. This is all that's required to get a working sitemap with Gatsby! By default, the generated sitemap path is /sitemap.xml and will include all of your site’s pages, but of course the plugin exposes options to configure this default functionality.
 
 ### Additional Modification

packages/gatsby-plugin-sitemap/src/internals.js

@@ -28,9 +28,14 @@ export const runQuery = (handler, query, excludes, pathPrefix) =>
       return page
     })
 
-    if (!r.data.site.siteMetadata.siteUrl) {
+    // siteUrl Validation
+    if (
+      !r.data.site.siteMetadata.siteUrl ||
+      r.data.site.siteMetadata.siteUrl == null ||
+      r.data.site.siteMetadata.siteUrl.trim().length == 0
+    ) {
       throw new Error(
-        `SiteMetaData 'siteUrl' property is required. Check out the documentation to see a working example: https://www.gatsbyjs.org/packages/gatsby-plugin-sitemap/#how-to-use`
+        `SiteMetaData 'siteUrl' property is required and cannot be left empty. Check out the documentation to see a working example: https://www.gatsbyjs.org/packages/gatsby-plugin-sitemap/#how-to-use`
       )
     }