Eliminate array resetting loop in _msg_support.c.em

[nvcyc]

Description

As comment in the following code suggested, there exists a part of the code that uses a less ideal way to reset the array.

rosidl_python/rosidl_generator_py/resource/_msg_support.c.em

Lines 589 to 608 in 01af609

	// clear the array, poor approach to remove potential default values
	Py_ssize_t length = PyObject_Length(field);
	if (-1 == length) {
	Py_DECREF(field);
	return NULL;
	}
	if (length > 0) {
	PyObject * pop = PyObject_GetAttrString(field, "pop");
	assert(pop != NULL);
	for (Py_ssize_t i = 0; i < length; ++i) {
	PyObject * ret = PyObject_CallFunctionObjArgs(pop, NULL);
	if (!ret) {
	Py_DECREF(pop);
	Py_DECREF(field);
	return NULL;
	}
	Py_DECREF(ret);
	}
	Py_DECREF(pop);
	}

This part can be improved by replacing with either

• PyObject * ret = PyObject_CallMethodNoArgs(field, "clear"); but this only works with Python 3.13 or higher (reference)
• PySequence_DelSlice(field, 0, length) that should work in any Python version

Motivation

This is discovered and suggested in a review from PR #250

Design / Implementation Considerations

No response

Additional Information

No response