Further simplify parse_compound_identifier (5/8)

This part changes behavior:
- Fail when no identifier is found.
- Avoid rewinding if EOF was hit right after the identifier.

tests/sqlparser_postgres.rs

@@ -103,6 +103,13 @@ fn parse_invalid_table_name() {
     assert!(ast.is_err());
 }
 
+#[test]
+fn parse_no_table_name() {
+    let mut parser = parser("");
+    let ast = parser.parse_tablename();
+    assert!(ast.is_err());
+}
+
 #[test]
 fn parse_insert_with_columns() {
     let sql = String::from("INSERT INTO public.customer (id, name, active) VALUES(1, 2, 3)");