Updates handling of `sha` being `nil`

* Adds helper for finding a sha so it can return nil

lib/gitlab/ci/config.rb

@@ -99,10 +99,18 @@ module Gitlab
         initial_config
       end
 
+      def find_sha(project)
+        branches = project&.repository&.branches || []
+
+        unless branches.empty?
+          project.repository.root_ref_sha
+        end
+      end
+
       def build_context(project:, sha:, user:, parent_pipeline:)
         Config::External::Context.new(
           project: project,
-          sha: sha || project&.repository&.root_ref_sha,
+          sha: sha || find_sha(project),
           user: user,
           parent_pipeline: parent_pipeline,
           variables: project&.predefined_variables&.to_runner_variables)

lib/gitlab/ci/lint.rb

@@ -21,7 +21,7 @@ module Gitlab
       def initialize(project:, current_user:, sha: nil)
         @project = project
         @current_user = current_user
-        @sha = sha || project.repository.commit.sha
+        @sha = sha || project.repository.commit&.sha
       end
 
       def validate(content, dry_run: false)

spec/requests/api/lint_spec.rb

@@ -406,6 +406,24 @@ RSpec.describe API::Lint do
       end
     end
 
+    context 'with an empty repository' do
+      let_it_be(:empty_project) { create(:project_empty_repo) }
+      let_it_be(:yaml_content) do
+        File.read(Rails.root.join('spec/support/gitlab_stubs/gitlab_ci.yml'))
+      end
+
+      before do
+        empty_project.add_developer(api_user)
+      end
+
+      it 'passes validation without errors' do
+        post api("/projects/#{empty_project.id}/ci/lint", api_user), params: { content: yaml_content }
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(json_response['valid']).to eq(true)
+        expect(json_response['errors']).to eq([])
+      end
+    end
+
     context 'when unauthenticated' do
       let_it_be(:api_user) { nil }