Fix creating an MR when a project approval rule has no approvers

Without this change, we assume the regular approval rule is actually an
"any_approver" approval rule. An MR can only have one of those, so we
get a constraint violation error when the regular approval rule is
converted.

It's odd that these rules are entirely valid in the backend, but making
them invalid at this stage feels a bit risky.

Changelog: fixed

ee/app/services/approval_rules/params_filtering_service.rb

@@ -53,7 +53,7 @@ module ApprovalRules
         rule_attributes[:user_ids] = provided_user_ids & visible_user_ids
       end
 
-      if rule_attributes[:group_ids].blank? && rule_attributes[:user_ids].blank?
+      if rule_attributes[:group_ids].blank? && rule_attributes[:user_ids].blank? && rule_attributes[:name].blank?
         rule_attributes[:rule_type] = :any_approver
         rule_attributes[:name] = ApprovalRuleLike::ALL_MEMBERS
       end

ee/changelogs/unreleased/273433-fix-handling-empty-approval-project-rules-when-creating-mr.yml

+---
+title: Fix creating an MR when a project approval rule has no approvers
+merge_request: 59671
+author:
+type: fixed

ee/spec/services/approval_rules/params_filtering_service_spec.rb

@@ -80,6 +80,22 @@ RSpec.describe ApprovalRules::ParamsFilteringService do
         let(:expected_groups) { [accessible_group] }
       end
 
+      # When a project approval rule is genuinely empty, it should not be converted
+      # an any_approver rule
+      context 'empty project approval rule' do
+        let(:approval_rules_attributes) { [{ name: 'Foo', user_ids: [], group_ids: [] }] }
+
+        it 'adds empty rule', :aggregate_failures do
+          rules = service.execute[:approval_rules_attributes]
+
+          expect(rules.size).to eq(1)
+          expect(rules[0]['name']).to eq('Foo')
+          expect(rules[0]['user_ids']).to be_empty
+          expect(rules[0]['group_ids']).to be_empty
+          expect(rules[0].key?('rule_type')).to be_falsy
+        end
+      end
+
       context 'inapplicable user defined rules' do
         let!(:source_rule) { create(:approval_project_rule, project: project) }
         let!(:another_source_rule) { create(:approval_project_rule, project: project) }
@@ -148,7 +164,7 @@ RSpec.describe ApprovalRules::ParamsFilteringService do
         let(:can_update_approvers?) { true }
         let(:approval_rules_attributes) do
           [
-            { user_ids: [], group_ids: [] }
+            { user_ids: [], group_ids: [], name: '' }
           ]
         end