build_release_candidate.sh fails if RC > 1 but there's no SVN directory on dist.apache.org

Currently the logic is "if RC \> 1 then delete the dist.apache.org staging directory".

Actually the logic should be "if the staging directory exists and we want to stage a new thing, delete the staging directory".

(the overall flow might change but this is surgical)

Imported from Jira [BEAM-6613](https://issues.apache.org/jira/browse/BEAM-6613). Original Jira may contain additional context.
Reported by: kenn.
This issue has child subcomponents which were not migrated over. See the original Jira for more information.
Subtask of issue #19755