FIX: problem when [] custom field is send (#13573)

Multiselect data can be saved but when all are removed then data are not cleared

Ajax function is removing an empty array from request data. In that case, we should change `[]` to `null`.

We need that empty values to properly empty data.

app/assets/javascripts/discourse/app/controllers/preferences/profile.js

@@ -86,21 +86,29 @@ export default Controller.extend({
       this.model.set("user_option.timezone", moment.tz.guess());
     },
 
-    save() {
-      this.set("saved", false);
-
+    _updateUserFields() {
       const model = this.model,
         userFields = this.userFields;
 
-      // Update the user fields
       if (!isEmpty(userFields)) {
         const modelFields = model.get("user_fields");
         if (!isEmpty(modelFields)) {
           userFields.forEach(function (uf) {
-            modelFields[uf.get("field.id").toString()] = uf.get("value");
+            const value = uf.get("value");
+            modelFields[uf.get("field.id").toString()] = isEmpty(value)
+              ? null
+              : value;
           });
         }
       }
+    },
+
+    save() {
+      this.set("saved", false);
+      const model = this.model;
+
+      // Update the user fields
+      this.send("_updateUserFields");
 
       return model
         .save(this.saveAttrNames)

app/assets/javascripts/discourse/tests/unit/controllers/preferences-profile-test.js

@@ -0,0 +1,41 @@
+import { discourseModule } from "discourse/tests/helpers/qunit-helpers";
+import { test } from "qunit";
+import EmberObject from "@ember/object";
+import User from "discourse/models/user";
+
+discourseModule("Unit | Controller | preferences/profile", function () {
+  test("prepare custom field data", function (assert) {
+    const controller = this.getController("preferences/profile", {
+      model: User.create({
+        id: 70,
+        second_factor_enabled: true,
+        is_anonymous: true,
+        user_fields: {
+          field_1: "1",
+          field_2: "2",
+          field_3: "3",
+        },
+      }),
+      currentUser: {
+        id: 1234,
+      },
+    });
+    controller.set("userFields", [
+      EmberObject.create({ value: "2", field: { id: "field_1" } }),
+      EmberObject.create({ value: null, field: { id: "field_2" } }),
+      EmberObject.create({ value: [], field: { id: "field_3" } }),
+    ]);
+    controller.send("_updateUserFields");
+    assert.equal(
+      controller.model.user_fields.field_1,
+      "2",
+      "updates string value"
+    );
+    assert.equal(controller.model.user_fields.field_2, null, "updates null");
+    assert.equal(
+      controller.model.user_fields.field_3,
+      null,
+      "updates empty array as null"
+    );
+  });
+});