Don't overwrite user's description by default template description

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/220941

**Problem**

When a project has a default description template for merge
requests. Then, it is not possible to set a custom template via push
options or query parameters.

**Solution**

Don't use a default description template when the description is
already defined.

ee/app/services/ee/merge_requests/build_service.rb

@@ -4,15 +4,16 @@ module EE
   module MergeRequests
     module BuildService
       def assign_title_and_description
-        super
-
         assign_description_from_template
+
+        super
       end
 
       # Set MR description based on project template
       def assign_description_from_template
         return unless target_project.feature_available?(:issuable_default_templates) &&
-            target_project.merge_requests_template.present?
+                      target_project.merge_requests_template.present? &&
+                      merge_request.description.blank?
 
         merge_request.description = target_project.merge_requests_template
 

ee/changelogs/unreleased/220941_fix_description_for_push_options.yml

+---
+title: "Don't overwrite user's description by default template description"
+merge_request: 40609
+author:
+type: fixed

ee/spec/services/merge_requests/build_service_spec.rb

@@ -40,6 +40,14 @@ RSpec.describe MergeRequests::BuildService do
       it 'does not set the MR description from template' do
         expect(merge_request.description).not_to eq(template)
       end
+
+      context 'when description is provided' do
+        let(:description) { 'Description' }
+
+        it "sets the user's description" do
+          expect(merge_request.description).to eq(description)
+        end
+      end
     end
 
     context 'issuable default templates feature available' do
@@ -50,6 +58,14 @@ RSpec.describe MergeRequests::BuildService do
       it 'sets the MR description from template' do
         expect(merge_request.description).to eq(template)
       end
+
+      context 'when description is provided' do
+        let(:description) { 'Description' }
+
+        it "prefers user's description to the default template" do
+          expect(merge_request.description).to eq(description)
+        end
+      end
     end
   end
 end