[substrait] Scalar subquery in select not supported

### Describe the bug

For certain queries with subqueries, DataFusion optimizes them as a LEFT join without condition (`ScalarSubqueryToJoin`).

A recent addition to require conditions for joins (https://github.com/apache/datafusion/pull/15334) has added the validation that causes problems in this type of query now:

```
---- cases::roundtrip_logical_plan::scalar_subquery_in_select stdout ----
Error: Plan("join condition should not be empty")
```

### To Reproduce

```sql
SELECT a, (SELECT MAX(b) FROM data2) as max_b FROM data
```
Returns error:

> Error: Plan("join condition should not be empty")

### Expected behavior

`Boolean(true)` in the filter:

```
Projection: data.a, max(data2.b) AS max_b
Left Join:
TableScan: data projection=[a]
Aggregate: groupBy=[[]], aggr=[[max(data2.b)]]
TableScan: data2 projection=[b], partial_filters=[Boolean(true)]
```

### Additional context

_No response_