Don't send custom msg in suggestion when disabled

When the feature flag is off, the frontend should not
send any different parameter to the api

app/assets/javascripts/api.js

@@ -442,10 +442,11 @@ const Api = {
     });
   },
 
-  applySuggestion(id, message) {
+  applySuggestion(id, message = '') {
     const url = Api.buildUrl(Api.applySuggestionPath).replace(':id', encodeURIComponent(id));
+    const params = gon.features?.suggestionsCustomCommit ? { commit_message: message } : false;
 
-    return axios.put(url, { commit_message: message });
+    return axios.put(url, params);
   },
 
   applySuggestionBatch(ids) {

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

@@ -88,7 +88,12 @@ export default {
     applySuggestion(message) {
       if (!this.canApply) return;
       this.isApplyingSingle = true;
-      this.$emit('apply', this.applySuggestionCallback, message);
+
+      this.$emit(
+        'apply',
+        this.applySuggestionCallback,
+        gon.features?.suggestionsCustomCommit ? message : undefined,
+      );
     },
     applySuggestionCallback() {
       this.isApplyingSingle = false;

changelogs/unreleased/300209-fix-suggestions-bad-request.yml

+---
+title: Fix bad request error when applying suggestions
+merge_request:
+author:
+type: fixed