🐛 Fixed 500 error on /rss/ when a post has no content

refs #10679

- this was a regression introduced after extracting our url-utils package
- `urlUtils.htmlRelativeToAbsolute(html)` will return `null` rather than an empty string if `html` is explicitly passed in as `null`

core/frontend/services/rss/generate-feed.js

@@ -23,7 +23,8 @@ generateTags = function generateTags(data) {
 
 generateItem = function generateItem(post, secure) {
     const itemUrl = urlService.getUrlByResourceId(post.id, {secure, absolute: true});
-    const htmlContent = cheerio.load(urlUtils.htmlRelativeToAbsolute(post.html, itemUrl, {secure}), {decodeEntities: false});
+    const html = post.html ? urlUtils.htmlRelativeToAbsolute(post.html, itemUrl, {secure}) : '';
+    const htmlContent = cheerio.load(html, {decodeEntities: false});
     const item = {
         title: post.title,
         // @TODO: DRY this up with data/meta/index & other excerpt code