fix git commit "nothing to commit" error

appveyor.yml

@@ -16,8 +16,7 @@ build_script:
 on_success:
   - cd docs
   - git add .
-  - git commit -m "CI Updates"
-  - git push -u origin master
+  - git diff --quiet --exit-code --cached || git commit -m "CI Updates" || git push -u origin master
 #on_finish:
 #  - ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))