Unparser emits empty SELECT list for empty Projection nodes, ignoring supports_empty_select_list #27332

Describe the bug

Unparsing the optimized plan of a simple count(*) query with a filter produces SELECT FROM ... (an empty select list), even for dialects whose supports_empty_select_list() is false. Engines that don't accept an empty select list reject the generated SQL. For example, BigQuery fails with:

SELECT list must not be empty

The column used in the filter isn't needed above the Filter, so optimize_projections prunes it and leaves a Projection with no expressions:

Projection: count(Int64(1)) AS count(*)
  Aggregate: groupBy=[[]], aggr=[[count(Int64(1))]]
    Projection:
      Filter: t.id > Int32(10)
        TableScan: t projection=[id]

#19221 (closing #18991) added Dialect::supports_empty_select_list and a SELECT 1 fallback, but only the TableScan path checks it (the case where the scan has projection=[], as in select count(*) from t with no filter). An empty Projection node goes through reconstruct_select_statement, which uses p.expr as is.

To Reproduce

use datafusion::prelude::SessionContext;
use datafusion::sql::unparser::plan_to_sql;

let ctx = SessionContext::new();
ctx.sql("create table t (id int)").await?.collect().await?;
let plan = ctx
    .sql("select count(*) from t where id > 10")
    .await?
    .into_optimized_plan()?;
println!("{}", plan_to_sql(&plan)?);

Output (default dialect):

SELECT count(1) AS "count(*)" FROM (SELECT FROM t WHERE (t.id > 10))

BigQueryDialect produces the same thing with backtick quoting.

Expected behavior

An empty Projection should get the same dialect-dependent fallback as an empty TableScan projection:

• Dialects with supports_empty_select_list() == false (default, BigQuery, ...):

  SELECT count(1) AS "count(*)" FROM (SELECT 1 FROM t WHERE (t.id > 10))

• Dialects that support it (PostgreSQL): unchanged, SELECT FROM ...

Additional context

The fix is small: in reconstruct_select_statement (datafusion/sql/src/unparser/plan.rs), use self.empty_projection_fallback() when p.expr is empty. I'll open a PR with the fix and tests.