.github/workflows/doc-build-pdf.yml: Fix get_ci_fixes when there are no fixes

Author: Matthias Koeppe

.github/workflows/doc-build-pdf.yml

@@ -35,7 +35,9 @@ jobs:
       - name: Store CI fixes in upstream artifact
         run: |
           mkdir -p upstream
-          git format-patch --stdout test_base > upstream/ci_fixes.patch
+          if git format-patch --stdout test_base > ci_fixes.patch; then
+            cp ci_fixes.patch upstream/
+          fi
       - uses: actions/upload-artifact@v3
         with:
           path: upstream