Fixed #31910 -- Fixed crash of GIS aggregations over subqueries.

Regression was introduced by fff5186 but was due a long standing issue.

AggregateQuery was abusing Query.subquery: bool by stashing its
compiled inner query's SQL for later use in its compiler which made
select_format checks for Query.subquery wrongly assume the provide
query was a subquery.

This patch prevents that from happening by using a dedicated
inner_query attribute which is compiled at a later time by
SQLAggregateCompiler.

Moving the inner query's compilation to SQLAggregateCompiler.compile
had the side effect of addressing a long standing issue with
aggregation subquery pushdown which prevented converters from being
run. This is now fixed as the aggregation_regress adjustments
demonstrate.

Refs #25367.

Thanks Eran Keydar for the report.

tests/aggregation_regress/tests.py

@@ -974,7 +974,7 @@ class AggregationTests(TestCase):
     def test_empty_filter_aggregate(self):
         self.assertEqual(
             Author.objects.filter(id__in=[]).annotate(Count("friends")).aggregate(Count("pk")),
-            {"pk__count": None}
+            {"pk__count": 0}
         )
 
     def test_none_call_before_aggregate(self):

tests/gis_tests/geoapp/tests.py

@@ -12,6 +12,7 @@ from django.core.management import call_command
 from django.db import DatabaseError, NotSupportedError, connection
 from django.db.models import F, OuterRef, Subquery
 from django.test import TestCase, skipUnlessDBFeature
+from django.test.utils import CaptureQueriesContext
 
 from ..utils import (
     mariadb, mysql, oracle, postgis, skipUnlessGISLookup, spatialite,
@@ -593,6 +594,19 @@ class GeoQuerySetTest(TestCase):
         qs = City.objects.filter(name='NotACity')
         self.assertIsNone(qs.aggregate(Union('point'))['point__union'])
 
+    @skipUnlessDBFeature('supports_union_aggr')
+    def test_geoagg_subquery(self):
+        ks = State.objects.get(name='Kansas')
+        union = GEOSGeometry('MULTIPOINT(-95.235060 38.971823)')
+        # Use distinct() to force the usage of a subquery for aggregation.
+        with CaptureQueriesContext(connection) as ctx:
+            self.assertIs(union.equals(
+                City.objects.filter(point__within=ks.poly).distinct().aggregate(
+                    Union('point'),
+                )['point__union'],
+            ), True)
+        self.assertIn('subquery', ctx.captured_queries[0]['sql'])
+
     @unittest.skipUnless(
         connection.vendor == 'oracle',
         'Oracle supports tolerance parameter.',