Fix Error 500 when browsing projects with no HEAD

Steps to reproduce:
1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Closes https://github.com/gitlabhq/gitlabhq/issues/9484

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.14.0 (unreleased)
+  - Fix Error 500 when browsing projects with no HEAD (Stan Hu)
   - Fix full screen mode for snippet comments (Daniel Gerhardt)
   - Fix 404 error in files view after deleting the last file in a repository (Stan Hu)
   - Fix label read access for unauthenticated users (Daniel Gerhardt)

app/helpers/projects_helper.rb

@@ -278,7 +278,8 @@ module ProjectsHelper
   end
 
   def readme_cache_key
-    [@project.id, @project.commit.sha, "readme"].join('-')
+    sha = @project.commit.try(:sha) || 'nil'
+    [@project.id, sha, "readme"].join('-')
   end
 
   def round_commit_count(project)

spec/helpers/projects_helper_spec.rb

@@ -22,7 +22,7 @@ describe ProjectsHelper do
 
     let(:user) { create(:user) }
 
-    it "returns false if there are no approipriate permissions" do
+    it "returns false if there are no appropriate permissions" do
       allow(helper).to receive(:can?) { false }
 
       expect(helper.can_change_visibility_level?(project, user)).to be_falsey
@@ -52,4 +52,22 @@ describe ProjectsHelper do
       end
     end
   end
+
+  describe "readme_cache_key" do
+    let(:project) { create(:project) }
+
+    before do
+      helper.instance_variable_set(:@project, project)
+    end
+
+    it "returns a valid cach key" do
+      expect(helper.send(:readme_cache_key)).to eq("#{project.id}-#{project.commit.id}-readme")
+    end
+
+    it "returns a valid cache key if HEAD does not exist" do
+      allow(project).to receive(:commit) { nil }
+
+      expect(helper.send(:readme_cache_key)).to eq("#{project.id}-nil-readme")
+    end
+  end
 end