Fix, only auto-save inputs when value is present

This ensures that when another admin is filling a field and the current
admin has a local auto-save restored with an empty value, the current
admin's auto-save will not clear the other admin's value.

app/javascript/controllers/admin/auto_save_controller.js

@@ -22,10 +22,10 @@ export default class extends Controller {
     for (var pair of form.entries()) {
       if (pair[0] != "authenticity_token") {
         let editor = this.formTarget.querySelector(`[name='${pair[0]}'] ~ trix-editor`)
-        if (editor) {
+        if (editor && !editor.editor.getDocument().isEmpty()) {
           data.push([pair[0], JSON.stringify(editor.editor)])
         }
-        else {
+        else if (pair[1]) {
           data.push([pair[0], pair[1]])
         }
       }