Refs #10929 -- Deprecated forced empty result value for PostgreSQL aggregates.

This deprecates forcing a return value for ArrayAgg, JSONBAgg, and
StringAgg when there are no rows in the query. Now that we have a
``default`` argument for aggregates, we want to revert to returning the
default of ``None`` which most aggregate functions return and leave it
up to the user to decide what they want to be returned by default.

django/contrib/postgres/aggregates/general.py

@@ -1,5 +1,8 @@
+import warnings
+
 from django.contrib.postgres.fields import ArrayField
 from django.db.models import Aggregate, BooleanField, JSONField, Value
+from django.utils.deprecation import RemovedInDjango50Warning
 
 from .mixins import OrderableAggMixin
 
@@ -8,20 +11,44 @@ __all__ = [
 ]
 
 
-class ArrayAgg(OrderableAggMixin, Aggregate):
+# RemovedInDjango50Warning
+NOT_PROVIDED = object()
+
+
+class DeprecatedConvertValueMixin:
+    def __init__(self, *expressions, default=NOT_PROVIDED, **extra):
+        if default is NOT_PROVIDED:
+            default = None
+            self._default_provided = False
+        else:
+            self._default_provided = True
+        super().__init__(*expressions, default=default, **extra)
+
+    def convert_value(self, value, expression, connection):
+        if value is None and not self._default_provided:
+            warnings.warn(self.deprecation_msg, category=RemovedInDjango50Warning)
+            return self.deprecation_value
+        return value
+
+
+class ArrayAgg(DeprecatedConvertValueMixin, OrderableAggMixin, Aggregate):
     function = 'ARRAY_AGG'
     template = '%(function)s(%(distinct)s%(expressions)s %(ordering)s)'
     allow_distinct = True
 
+    # RemovedInDjango50Warning
+    deprecation_value = property(lambda self: [])
+    deprecation_msg = (
+        'In Django 5.0, ArrayAgg() will return None instead of an empty list '
+        'if there are no rows. Pass default=None to opt into the new behavior '
+        'and silence this warning or default=Value([]) to keep the previous '
+        'behavior.'
+    )
+
     @property
     def output_field(self):
         return ArrayField(self.source_expressions[0].output_field)
 
-    def convert_value(self, value, expression, connection):
-        if value is None and self.default is None:
-            return []
-        return value
-
 
 class BitAnd(Aggregate):
     function = 'BIT_AND'
@@ -41,28 +68,36 @@ class BoolOr(Aggregate):
     output_field = BooleanField()
 
 
-class JSONBAgg(OrderableAggMixin, Aggregate):
+class JSONBAgg(DeprecatedConvertValueMixin, OrderableAggMixin, Aggregate):
     function = 'JSONB_AGG'
     template = '%(function)s(%(distinct)s%(expressions)s %(ordering)s)'
     allow_distinct = True
     output_field = JSONField()
 
-    def convert_value(self, value, expression, connection):
-        if value is None and self.default is None:
-            return '[]'
-        return value
+    # RemovedInDjango50Warning
+    deprecation_value = '[]'
+    deprecation_msg = (
+        "In Django 5.0, JSONBAgg() will return None instead of an empty list "
+        "if there are no rows. Pass default=None to opt into the new behavior "
+        "and silence this warning or default=Value('[]') to keep the previous "
+        "behavior."
+    )
 
 
-class StringAgg(OrderableAggMixin, Aggregate):
+class StringAgg(DeprecatedConvertValueMixin, OrderableAggMixin, Aggregate):
     function = 'STRING_AGG'
     template = '%(function)s(%(distinct)s%(expressions)s %(ordering)s)'
     allow_distinct = True
 
+    # RemovedInDjango50Warning
+    deprecation_value = ''
+    deprecation_msg = (
+        "In Django 5.0, StringAgg() will return None instead of an empty "
+        "string if there are no rows. Pass default=None to opt into the new "
+        "behavior and silence this warning or default=Value('') to keep the "
+        "previous behavior."
+    )
+
     def __init__(self, expression, delimiter, **extra):
         delimiter_expr = Value(str(delimiter))
         super().__init__(expression, delimiter_expr, **extra)
-
-    def convert_value(self, value, expression, connection):
-        if value is None and self.default is None:
-            return ''
-        return value