Fix DDL generation in case of an empty arguments function. Fixes #1690.

src/ast/ddl.rs

@@ -2011,6 +2011,8 @@ impl fmt::Display for CreateFunction {
         )?;
         if let Some(args) = &self.args {
             write!(f, "({})", display_comma_separated(args))?;
+        } else {
+            write!(f, "()")?;
         }
         if let Some(return_type) = &self.return_type {
             write!(f, " RETURNS {return_type}")?;

tests/sqlparser_postgres.rs

@@ -3802,6 +3802,7 @@ fn parse_create_function_detailed() {
     pg_and_generic().verified_stmt("CREATE OR REPLACE FUNCTION add(a INTEGER, IN b INTEGER = 1) RETURNS INTEGER LANGUAGE SQL STABLE PARALLEL UNSAFE RETURN a + b");
     pg_and_generic().verified_stmt("CREATE OR REPLACE FUNCTION add(a INTEGER, IN b INTEGER = 1) RETURNS INTEGER LANGUAGE SQL STABLE CALLED ON NULL INPUT PARALLEL UNSAFE RETURN a + b");
     pg_and_generic().verified_stmt(r#"CREATE OR REPLACE FUNCTION increment(i INTEGER) RETURNS INTEGER LANGUAGE plpgsql AS $$ BEGIN RETURN i + 1; END; $$"#);
+    pg_and_generic().verified_stmt(r#"CREATE OR REPLACE FUNCTION no_arg() RETURNS VOID LANGUAGE plpgsql AS $$ BEGIN DELETE FROM my_table; END; $$"#);
 }
 #[test]
 fn parse_incorrect_create_function_parallel() {