AVG(nulls) returns 0 rather than NULL

[alamb]

Describe the bug
Calling the avg function with all null values should be null, not 0

To Reproduce
with datafusion-cli

❯ create table foo as values (null::bigint, 1);
0 rows in set. Query took 0.002 seconds.
❯ select avg(column1) from foo group by column2;
+------------------+
| AVG(foo.column1) |
+------------------+
| 0                |
+------------------+

Expected behavior
AVG of all nulls should be null

postgres=# create table foo as values (null::bigint, 1);
SELECT 1
postgres=# select avg(column1) from foo group by column2;
 avg 
-----
    
(1 row)

Additional context

Found when upgrading IOx in https://github.com/influxdata/influxdb_iox/pull/6639

I think the issue is that there was a bug in the Row evaluator that was not being used until after #4924 was merged

I have a fix and will get it up shortly