fmt: fix ?void format error, add _keep.vv test

vlib/v/fmt/tests/void_optional_keep.vv

@@ -0,0 +1,9 @@
+fn tt() ? {
+	return error('error')
+}
+
+fn main() {
+	tt() or {
+		panic('$err')
+	}
+}

vlib/v/table/atypes.v

@@ -707,8 +707,12 @@ pub fn (table &Table) type_to_str(t Type) string {
 		res = strings.repeat(`&`, nr_muls) + res
 	}
 	if t.flag_is(.optional) {
+		if sym.kind == .void {
+			res = '?'
+		} else {
 			res = '?' + res
 		}
+	}
 	/*
 	if res.starts_with(cur_mod +'.') {
 	res = res[cur_mod.len+1.. ]