🐛 Koenig - Fixed rich-text captions sometimes losing spaces

refs https://github.com/TryGhost/Ghost/issues/9724
- `clean-basic-html` was being overzealous with it's empty element removal, often contenteditable (especially when pasting) will result in HTML such as `<span>&nbsp;</span>` which was being completely removed
- if an empty element has any spaces in it, replace the element with a textNode containing a single space

lib/koenig-editor/addon/helpers/clean-basic-html.js

@@ -19,7 +19,13 @@ export function cleanBasicHtml(html = '') {
 
         doc.body.querySelectorAll('*').forEach((element) => {
             if (!element.textContent.trim()) {
-                element.remove();
+                if (element.textContent.length > 0) {
+                    // keep a single space to avoid collapsing spaces
+                    let space = document.createTextNode(' ');
+                    element.replaceWith(space);
+                } else {
+                    element.remove();
+                }
             }
         });