fix dropping function with empty brackets

Signed-off-by: Runji Wang <[email protected]>

Author: wangrunji0408

e2e_test/udf/python.slt

@@ -177,6 +177,10 @@ drop function gcd;
 statement ok
 drop function gcd(int, int);
 
+# foo() is different from foo.
+statement error function not found
+drop function gcd();
+
 # Drop a function without arguments. Now the function name is unique.
 statement ok
 drop function gcd;

src/sqlparser/src/parser.rs

@@ -2108,7 +2108,7 @@ impl Parser {
 
         let args = if self.consume_token(&Token::LParen) {
             if self.consume_token(&Token::RParen) {
-                None
+                Some(vec![])
             } else {
                 let args = self.parse_comma_separated(Parser::parse_function_arg)?;
                 self.expect_token(&Token::RParen)?;