Clarify Aggregate.empty_result_set_value docs.

[Claude Paroz]

While translating the new docs, I struggled to understand the meaning of this sentence about Aggregate.empty_result_set_value:

Override empty_result_set_value to None since most aggregate functions result in NULL when applied to an empty result set.

It's totally possible my limited English prevents me understanding this, in that case, maybe someone may explains that to me?

​https://docs.djangoproject.com/en/4.0/ref/models/expressions/#django.db.models.Aggregate.empty_result_set_value

[Tim Graham]

Looking at 9f3cce172f6913c5ac74272fa5fc07f847b4e112 where it was introduced, the language seems to resemble a code comment explaining why Aggregate_aggregate_value is set to None. It's unclear to me if this information is useful to Django users. At the least, it seems a rephrasing is appropriate ("Override" to "Overrides", perhaps).

[Mariusz Felisiak]

Aggregate.empty_result_set_value defaults to None which is fine for most (all?) aggregates, so I'd clarify this in docs, e.g.:

docs/ref/models/expressions.txt

@@ -418 +418 @@
 
         .. versionadded:: 4.0
 
-        Override :attr:`~django.db.models.Expression.empty_result_set_value` to
-        ``None`` since most aggregate functions result in ``NULL`` when applied
-        to an empty result set.
+        Defaults to ``None`` since most aggregate functions result in ``NULL``
+        when applied to an empty result set.
 
 The ``expressions`` positional arguments can include expressions, transforms of
 the model field, or the names of model fields. They will be converted to a

[Claude Paroz]

Ah thanks, now I understand (even the original sentence!). The new proposal is a lot better!

[Mariusz Felisiak]

​PR

[GitHub <noreply@…>]

In 4400d856:

Fixed #33391 -- Clarified Aggregate.empty_result_set_value docs.

[Mariusz Felisiak <felisiak.mariusz@…>]

In fe59bf2:

[4.0.x] Fixed #33391 -- Clarified Aggregate.empty_result_set_value docs.

Backport of 4400d8568ad5695c46e8de45635a82a27a26b871 from main