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ARROW-9911: [Rust][DataFusion] SELECT <expression> with no FROM clause should produce a single row of output

This PR makes datafusion will output evaluation result of `SELECT` expression without `FROM` clause.

prev:
```
> select 1 ;
0 rows in set. Query took 0 seconds.
```

now:
```
> select 1;
+----------+
| Int64(1) |
+----------+
| 1        |
+----------+
1 rows in set. Query took 0 seconds.
```

Previously, datafusion will generate an `EmptyExec` as input for outer `ProjectionExec`.  `EmptyExec` will return an empty stream when executing. And `ProjectionExec` won't evaluate its expression as it gets nothing from its "input". So I made a stream with one null value as a placeholder to feed it instead of `EmptyExec`.

Signed-off-by: wayne <i@waynest.com>

Closes #8662 from waynexia/ARROW-9911

Authored-by: wayne <i@waynest.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>