Ignore nil labels with issuable creation

If a user provides `nil` as a parameter for `labels`, creating an issue,
merge request, or epic would fail with a 500 error. This happened
because Rails would attempt to iterate on the association.

This issue was exposed when we upgraded Grape in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/27276 because
`nil` values were not coerced to empty Arrays automatically.

`IssuableBaseService#label_ids_to_filter` attempts to map the `labels`
value to `label_ids` and clears out the `labels` key. However, this
isn't done if the `labels` value is `nil`.

To avoid this, we should always clear out the key after
`label_ids_to_filter` completes.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/215936

app/services/issuable_base_service.rb

@@ -91,6 +91,8 @@ class IssuableBaseService < BaseService
     elsif params[label_key]
       params[label_id_key] = labels_service.find_or_create_by_titles(label_key, find_only: find_only).map(&:id)
     end
+
+    params.delete(label_key) if params[label_key].nil?
   end
 
   def filter_labels_in_param(key)

spec/services/issues/create_service_spec.rb

@@ -110,6 +110,31 @@ describe Issues::CreateService do
         end
       end
 
+      context 'when labels is nil' do
+        let(:opts) do
+          { title: 'Title',
+            description: 'Description',
+            labels: nil }
+        end
+
+        it 'does not assign label' do
+          expect(issue.labels).to be_empty
+        end
+      end
+
+      context 'when labels is nil and label_ids is present' do
+        let(:opts) do
+          { title: 'Title',
+            description: 'Description',
+            labels: nil,
+            label_ids: labels.map(&:id) }
+        end
+
+        it 'assigns group labels' do
+          expect(issue.labels).to match_array labels
+        end
+      end
+
       context 'when milestone belongs to different project' do
         let(:milestone) { create(:milestone) }