Fix force_remove_source_branch not working in API

When an MR is merged via the API without `should_remove_source_branch`
set, the branch is always retained, even if `force_remove_source_branch`
is set to true in the MR merge params. This was happening because the
API always passed a `should_remove_source_branch` with a `nil` value,
and `MergeService` interpreted this as `false`.

To fix this, we strip `nil` values from the `merge_params`.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/219991

changelogs/unreleased/sh-fix-issue-219991.yml

+---
+title: Fix force_remove_source_branch not working in API
+merge_request: 33804
+author:
+type: fixed

lib/api/merge_requests.rb

@@ -477,7 +477,7 @@ module API
           squash_commit_message: params[:squash_commit_message],
           should_remove_source_branch: params[:should_remove_source_branch],
           sha: params[:sha] || merge_request.diff_head_sha
-        )
+        ).compact
 
         if immediately_mergeable
           ::MergeRequests::MergeService

spec/requests/api/merge_requests_spec.rb

@@ -2029,6 +2029,34 @@ describe API::MergeRequests do
       end
     end
 
+    context "with a merge request that has force_remove_source_branch enabled" do
+      let(:source_repository) { merge_request.source_project.repository }
+      let(:source_branch) { merge_request.source_branch }
+
+      before do
+        merge_request.update(merge_params: { 'force_remove_source_branch' => true })
+      end
+
+      it 'removes the source branch' do
+        put(
+          api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user)
+        )
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(source_repository.branch_exists?(source_branch)).to be_falsy
+      end
+
+      it 'does not remove the source branch' do
+        put(
+          api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user),
+          params: { should_remove_source_branch: false }
+        )
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(source_repository.branch_exists?(source_branch)).to be_truthy
+      end
+    end
+
     context "performing a ff-merge with squash" do
       let(:merge_request) { create(:merge_request, :rebased, source_project: project, squash: true) }