FIX: Don't require fields required on sign-up when updating fields (#27888)

### What is the problem?

We have recently added a new option to add user fields required for existing users. This is in contrast to requiring fields only on sign-up.

This revealed an existing problem. Consider the following:

1. User A signs up.
2. Admin adds a new user field required on sign-up. (Should not apply to User A since they already signed up.)
3. User A tries to update their profile.

**Expected behaviour:**

No problem.

**Actual behaviour:**

User A receives an error saying they didn't fill up all required fields.

### How does this fix it?

When updating profile, we only check that required fields that are "for all users" are filled. Additionally, we check that fields that were required on sign-up and have previously been filled are not blanked out.

app/controllers/users_controller.rb

@@ -220,7 +220,12 @@ class UsersController < ApplicationController
         value = nil if value === "false"
         value = value[0...UserField.max_length] if value
 
-        if value.blank? && field.required?
+        if value.blank? &&
+             (
+               field.for_all_users? ||
+                 field.on_signup? &&
+                   user.custom_fields["#{User::USER_FIELD_PREFIX}#{field_id}"].present?
+             )
           return render_json_error(I18n.t("login.missing_user_field"))
         end
         attributes[:custom_fields]["#{User::USER_FIELD_PREFIX}#{field.id}"] = value

spec/requests/users_controller_spec.rb

@@ -1528,6 +1528,20 @@ RSpec.describe UsersController do
             end.not_to change { user1.reload.user_fields[field_id] }
           end
 
+          it "value is required only on sign-up" do
+            user_field.on_signup!
+
+            expect do
+              put update_user_url, params: { user_fields: { field_id => "" } }
+            end.to change { user1.reload.user_fields[field_id] }.from(nil).to("")
+
+            put update_user_url, params: { user_fields: { field_id => valid_options } }
+
+            expect do
+              put update_user_url, params: { user_fields: { field_id => "" } }
+            end.not_to change { user1.reload.user_fields[field_id] }
+          end
+
           it "value can nil or empty if the field is not required" do
             put update_user_url, params: { user_fields: { field_id => valid_options } }