Return empty string instead of NULL in serialize().

Modifiy its return type accordingly and arginfo.

Author: Girgias

ext/standard/basic_functions.stub.php

@@ -1295,7 +1295,7 @@ function var_export($value, bool $return = false): ?string {}
 function debug_zval_dump($value, ...$value): void {}
 
 /** @param mixed $value */
-function serialize($value): ?string {}
+function serialize($value): string {}
 
 /** @return mixed */
 function unserialize(string $value, array $options = []) {}

ext/standard/basic_functions_arginfo.h

@@ -1940,7 +1940,7 @@ ZEND_END_ARG_INFO()
 
 #define arginfo_debug_zval_dump arginfo_var_dump
 
-ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_serialize, 0, 1, IS_STRING, 1)
+ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_serialize, 0, 1, IS_STRING, 0)
 	ZEND_ARG_INFO(0, value)
 ZEND_END_ARG_INFO()
 

ext/standard/var.c

@@ -1138,7 +1138,7 @@ PHPAPI void php_var_serialize_destroy(php_serialize_data_t d) {
 	}
 }
 
-/* {{{ proto string|null serialize(mixed variable)
+/* {{{ proto string serialize(mixed variable)
    Returns a string representation of variable (which can later be unserialized) */
 PHP_FUNCTION(serialize)
 {
@@ -1162,7 +1162,7 @@ PHP_FUNCTION(serialize)
 	if (buf.s) {
 		RETURN_NEW_STR(buf.s);
 	} else {
-		RETURN_NULL();
+		RETURN_EMPTY_STRING();
 	}
 }
 /* }}} */