cgen: fix method call checking against none

[felipensp]

Fix #20711

struct Foo {
	x int
}
struct Bar {
	x int
}
type Foobar = Foo | Bar
struct Foobars {
	m map[string]Foobar
}
fn (f &Foobars) find_foobar(name string) ?Foobar {
	return f.m[name] or { return none }
}
fn (mut f Foobars) is_known(name string) bool {
	return f.find_foobar(name) != none
}

fn main() {
	mut foobars := Foobars{
		m: map[string]Foobar{}
	}
	println(foobars.is_known('deadbeef'))
}

[spytheman]

Excellent work.