Merge branch 'ph/325513/fixedJSErrorSuggestionPreview' into 'master'

Fixes Vue prop error when previewing suggestions

See merge request gitlab-org/gitlab!69554

app/assets/javascripts/vue_shared/components/markdown/suggestion_diff_header.vue

@@ -39,7 +39,8 @@ export default {
     },
     defaultCommitMessage: {
       type: String,
-      required: true,
+      required: false,
+      default: null,
     },
     inapplicableReason: {
       type: String,