Physical-plan counterpart for DF apache#11256.

In apache#11256 it started inferring, for logical plans only,
that count() aggregation does not return nulls.
However, it did not introduce the counterpart inference for physical plans.
Somehow, this discrepancy did not get detected by DF tests, but we run into it with, e.g.
`sdf run -q "SELECT count(*) FROM (VALUES (3, 'c')) as T(c, x) GROUP BY c "`

The fix here is ad hoc.
We should keep an eye on apache#11274 where the goal seems to make UDAFs to communicate nullability of their result.  If done right, that should have affect both logical and physical plans.

datafusion/physical-expr-common/src/aggregate/mod.rs

@@ -534,7 +534,8 @@ impl AggregateExpr for AggregateFunctionExpr {
     }
 
     fn field(&self) -> Result<Field> {
-        Ok(Field::new(&self.name, self.data_type.clone(), true))
+        let nullable = self.fun.name() != "count";
+        Ok(Field::new(&self.name, self.data_type.clone(), nullable))
     }
 
     fn create_accumulator(&self) -> Result<Box<dyn Accumulator>> {