Export Rules as YAML MacOS filename issue

### Problem
Refer to the issue noted here: https://github.com/cdisc-org/conformance-rules-editor/issues/213#issuecomment-2032941237

"I believe this could be an issue specific to Mac OS. When files are created, they use a filename format like: RuleID.UUID.yml
If a rule is missing a RuleID (There are currently 4 such rules), the filename will begin with a dot. MacOS treats these as hidden files."

### Solution
The export functionality should use a placeholder for missing RuleIDs so that filenames do not start with periods.