Stop finding commit with empty ref

When `Projects::RefsController#switch` is requested without `ref`
and `path` params, internally, GitLab still tries to make a
request for `FindCommit` RPC with empty ref.

We don't need to do that as we won't be able to find a commit with
empty ref.

changelogs/unreleased/280795-stop-finding-commit-empty-ref.yml

+---
+title: Stop finding commit with empty ref
+merge_request: 47497
+author:
+type: fixed

lib/extracts_ref.rb

@@ -68,7 +68,7 @@ module ExtractsRef
 
     raise InvalidPathError if @ref.match?(/\s/)
 
-    @commit = @repo.commit(@ref)
+    @commit = @repo.commit(@ref) if @ref.present?
   end
   # rubocop:enable Gitlab/ModuleWithInstanceVariables
 

spec/lib/extracts_path_spec.rb

@@ -112,6 +112,19 @@ RSpec.describe ExtractsPath do
         end
       end
     end
+
+    context 'ref and path are nil' do
+      let(:params) { { path: nil, ref: nil } }
+
+      it 'does not set commit' do
+        expect(container.repository).not_to receive(:commit).with('')
+        expect(self).to receive(:render_404)
+
+        assign_ref_vars
+
+        expect(@commit).to be_nil
+      end
+    end
   end
 
   it_behaves_like 'extracts refs'

spec/lib/extracts_ref_spec.rb

@@ -18,6 +18,21 @@ RSpec.describe ExtractsRef do
     allow_any_instance_of(described_class).to receive(:repository_container).and_return(container)
   end
 
-  it_behaves_like 'assigns ref vars'
+  describe '#assign_ref_vars' do
+    it_behaves_like 'assigns ref vars'
+
+    context 'ref and path are nil' do
+      let(:params) { { path: nil, ref: nil } }
+
+      it 'does not set commit' do
+        expect(container.repository).not_to receive(:commit).with('')
+
+        assign_ref_vars
+
+        expect(@commit).to be_nil
+      end
+    end
+  end
+
   it_behaves_like 'extracts refs'
 end