fix(gatsby-plugin-sitemap): add meaningful error when siteUrl is miss…

…ing (gatsbyjs#13123)

## Description
Throw a more descriptive error when users do not supply a `siteUrl` property when using `gatsby-plugin-sitemap`

## Related Issues
Fixes gatsbyjs#12912

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

@@ -69,6 +69,20 @@ describe(`results using default settings`, () => {
 
       verifyUrlsExistInResults(urls, [`http://dummy.url${pathPrefix}/page-1`])
     })
+
+    it(`should fail when siteUrl is not set`, async () => {
+      const graphql = () =>
+        Promise.resolve(generateQueryResultsMock({ siteUrl: null }))
+      expect.assertions(1)
+
+      try {
+        await runQuery(graphql, ``, [], pathPrefix)
+      } catch (err) {
+        expect(err.message).toEqual(
+          expect.stringContaining(`SiteMetaData 'siteUrl' property is required`)
+        )
+      }
+    })
   }
 
   describe(`no path-prefix`, () => {

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

@@ -29,13 +29,17 @@ export const runQuery = (handler, query, excludes, pathPrefix) =>
       return page
     })
 
-    if (r.data.site.siteMetadata.siteUrl) {
-      // remove trailing slash of siteUrl
-      r.data.site.siteMetadata.siteUrl = withoutTrailingSlash(
-        r.data.site.siteMetadata.siteUrl
+    if (!r.data.site.siteMetadata.siteUrl) {
+      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`
       )
     }
 
+    // remove trailing slash of siteUrl
+    r.data.site.siteMetadata.siteUrl = withoutTrailingSlash(
+      r.data.site.siteMetadata.siteUrl
+    )
+
     return r.data
   })