skip when nothing to clean (#465)

Co-authored-by: Christopher Doris <github.com/cjdoris>

.github/workflows/docscleanup.yml

@@ -15,16 +15,15 @@ jobs:
 
       - name: Delete preview and history
         run: |
-            git config user.name "Documenter.jl"
-            git config user.email "[email protected]"
-            git rm -rf "previews/PR$PRNUM"
-            git commit -m "delete preview"
-            git branch gh-pages-new $(echo "delete history" | git commit-tree HEAD^{tree})
+            if [ -d "$DIR" ]; then
+              git config user.name "Documenter.jl"
+              git config user.email "[email protected]"
+              git rm -rf "$DIR"
+              git commit -m "delete preview"
+              git branch gh-pages-new $(echo "delete history" | git commit-tree HEAD^{tree})
+              git push --force origin gh-pages-new:gh-pages
+            fi
         env:
-            PRNUM: ${{ github.event.number }}
+            DIR: "previews/PR${{ github.event.number }}"
 
-      - name: Push changes
-        run: |
-            git push --force origin gh-pages-new:gh-pages
-            
 # Workflow copied from https://github.com/CliMA/TimeMachine.jl