Refs #26430 -- Re-introduced empty aggregation optimization.

The introduction of the Expression.empty_aggregate_value interface
allows the compilation stage to enable the EmptyResultSet optimization
if all the aggregates expressions implement it.

This also removes unnecessary RegrCount/Count.convert_value() methods.
Disabling the empty result set aggregation optimization when it wasn't
appropriate prevented None returned for a Count aggregation value.

Thanks Nick Pope for the review.

docs/ref/models/expressions.txt

@@ -410,6 +410,14 @@ The ``Aggregate`` API is as follows:
         allows passing a ``distinct`` keyword argument. If set to ``False``
         (default), ``TypeError`` is raised if ``distinct=True`` is passed.
 
+    .. attribute:: empty_aggregate_value
+
+        .. versionadded:: 4.0
+
+        Override :attr:`~django.db.models.Expression.empty_aggregate_value` 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
 string and used as the ``expressions`` placeholder within the ``template``.
@@ -950,6 +958,15 @@ calling the appropriate methods on the wrapped expression.
         in :class:`~django.db.models.expressions.Window`. Defaults to
         ``False``.
 
+    .. attribute:: empty_aggregate_value
+
+        .. versionadded:: 4.0
+
+        Tells Django which value should be returned when the expression is used
+        to :meth:`aggregate <django.db.models.query.QuerySet.aggregate>` over
+        an empty result set. Defaults to :py:data:`NotImplemented` which forces
+        the expression to be computed on the database.
+
     .. method:: resolve_expression(query=None, allow_joins=True, reuse=None, summarize=False, for_save=False)
 
         Provides the chance to do any pre-processing or validation of