Only use committed_at from CI when it is set

This is a small follow-up to #168 that I just noticed before cutting the
v1.0.5 release

Author: maxjacobson

lib/code_climate/test_reporter/git.rb

@@ -39,7 +39,9 @@ def head
         end
 
         def committed_at_from_ci
-          Ci.service_data[:committed_at].to_i
+          if (value = Ci.service_data[:committed_at])
+            value.to_i
+          end
         end
 
         def committed_at_from_git

spec/code_climate/test_reporter/git_spec.rb

@@ -90,6 +90,13 @@ def self.root
 
         expect(Git.committed_at_from_git_or_ci).to eq 1484768698
       end
+
+      it 'returns nil when there is neither' do
+        expect(Git).to receive(:committed_at_from_git).and_return(nil)
+        allow(Ci).to receive(:service_data).and_return({})
+
+        expect(Git.committed_at_from_git_or_ci).to be_nil
+      end
     end
 
   end