### Godot version

v4.0.beta13.official [caacade56]

### System information

Linux Ubuntu 20.04 with Unity desktop

### Issue description

When calling an undefined function in the shader editor with argument(s), such as `undefined_function(1)`, the error reads:

> Too many arguments for "undefined_function()" call. Expected at most 0 but received 1.

instead of the expected:

> No matching function found for: 'undefined_function'.

This is possibly a regression introduced in https://github.com/godotengine/godot/issues/18225, since the latter was focused on the arguments part.

To avoid such regression in the future, consider adding a unit test with this case (and, in fact, unit tests for the other signature errors too if there are not already).

### Steps to reproduce

1. Create a shader resource and open the shader editor
2. Enter `undefined_function(1);` in the fragment() definition

### Minimal reproduction project

N/A
identifier = StringName();
	TkPos pos = { 0, 0 };
	Token tk = _get_token();
	if (tk.type == TK_IDENTIFIER) {
		identifier = tk.text;
		pos = _get_tkpos();
		tk = _get_token();
	}
	if (tk.type == TK_CURSOR) {
		completion_type = p_type;
		completion_line = tk_line;
		completion_block = p_block;
		pos = _get_tkpos();
		tk = _get_token();
		if (tk.type == TK_IDENTIFIER) {
			identifier = identifier.operator String() + tk.text.operator String();
		} else {
			_set_tkpos(pos);
		}
		return true;
	} else if (identifier != StringName()) {
		_set_tkpos(pos);
	}
	return false;
}
bool ShaderLanguage::_is_operator_assign(Operator p_op) const {