N1QL: Avg on a field returns null as output when select of that field returns an empty set

cbq> select AVG( DISTINCT decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%';
{
"requestID": "6bcb6aae-6a49-4ab6-8344-d7eedb985b3c",
"signature":

{ "$1": "number" }

,
"results": [

{ "$1": null }

],
"status": "success",
"metrics":

{ "elapsedTime": "199.822094ms", "executionTime": "199.588874ms", "resultCount": 1, "resultSize": 34 }

}

while select for same:
cbq> select DISTINCT (decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%';
{
"requestID": "ccbed21e-a155-4af7-9511-f28c07c2116d",
"signature":

{ "decimal_field1": "json" }

,
"results": [
],
"status": "success",
"metrics":

{ "elapsedTime": "193.466108ms", "executionTime": "193.307897ms", "resultCount": 0, "resultSize": 0 }

}

Results in sql are also same:
mysql> select AVG( DISTINCT decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%' ;
--------------------------------

--------------------------------

--------------------------------
1 row in set (0.01 sec)

mysql> select DISTINCT (decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%';
Empty set (0.00 sec)

But above statements with group by are treated differently in mysql and n1ql as explained below:
mysql> select DISTINCT (decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%' GROUP BY primary_key_id ;
Empty set (0.00 sec)

mysql> select AVG( DISTINCT decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%' GROUP BY primary_key_id ;
Empty set (0.00 sec)

cbq> select AVG( DISTINCT decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%' GROUP BY primary_key_id ;
{
"requestID": "d6bb4c01-12df-47af-a024-1f02919ecf09",
"signature":

{ "$1": "number" }

,
"results": [

{ "$1": null }

],
"status": "success",
"metrics":

{ "elapsedTime": "158.715687ms", "executionTime": "158.532489ms", "resultCount": 1, "resultSize": 34 }

}

Above query should return an empty set with group by similar to:

cbq> select DISTINCT (decimal_field1 ) from simple_table where varchar_field1 LIKE 'aVIiIlSwoJ%' GROUP BY primary_key_id ;
{
"requestID": "778af305-c1ed-49d5-9f1b-4f9cedad03b5",
"signature":

{ "decimal_field1": "json" }

,
"results": [
{}
],
"status": "success",
"metrics":

{ "elapsedTime": "159.574506ms", "executionTime": "159.414275ms", "resultCount": 1, "resultSize": 2 }

}