Answer
Yes — a SQL-expression Report Rule action can use COALESCE across the two candidate values inside the bracketed SQL-expression syntax, structured as a select coalesce(<field-reference>, <current-user-email-token>) from dual expression (with each candidate value quoted as a literal) — this evaluates to the supplier's email when one is populated on the record, and falls back to the current user's own email automatically when it isn't, without needing separate report rules for each scenario.