Turn off error if no stale previews found in DocPreviewCleanup.yml

Instead of `error`-ing, set an environment variable that determines if the next step should be run.
Lack of stale PR previews is not a failure condition, just an early termination one.

Author: abhro

.github/workflows/DocPreviewCleanup.yml

@@ -62,15 +62,18 @@ jobs:
 
           if isempty(stale_previews)
               @info "No stale previews"
-              exit(1)
+              println(ENV["GITHUB_ENV"], "FOUND_STALE_PREVIEWS=false")
+              exit()
           end
+          println(ENV["GITHUB_ENV"], "FOUND_STALE_PREVIEWS=true")
 
           for pr in stale_previews
               path = joinpath("previews", "PR$pr")
               @info "Removing $path"
               run(`git rm -rf $path`)
           end
       - name: Push changes
+        if: ${{ env.FOUND_STALE_PREVIEWS == 'true' }}
         run: |
           git config user.name "Documenter.jl"
           git config user.email "[email protected]"