notmuch: default empty search to wildcard

notmuch/src/lib.rs

@@ -491,6 +491,8 @@ impl Notmuch {
         offset: usize,
         limit: usize,
     ) -> Result<SearchSummary, NotmuchError> {
+        let query = if query.is_empty() { "*" } else { query };
+
         let res = self.run_notmuch([
             "search",
             "--format=json",