Lexical: Added backspace handling for details

Allows more reliable removal of details block on backspace at first
child position with the details block.

resources/js/wysiwyg/lexical/rich-text/LexicalDetailsNode.ts

@@ -178,6 +178,10 @@ export class DetailsNode extends ElementNode {
         return true;
     }
 
+    canBeEmpty(): boolean {
+        return false;
+    }
+
 }
 
 export function $createDetailsNode() {