Mark Postgres aggregate return types as nullable (#498)

### What

As discussed internally, a particular quirk of SQL (or Postgres, at the
very least) is that, with the exception of `COUNT`, an aggregate of an
empty row set returns `NULL`[^1] (rather than, for example, returning
`0` for `SUM`). This means that all aggregate functions over scalar
types exposed by the Postgres connector should be labelled as nullable.

This PR addresses
[APG-131](https://hasurahq.atlassian.net/browse/APG-131).

NB: this will be a breaking change for anyone generating (for example)
types from our /schema output.

### How

This PR updates the `/schema` endpoint to mark these types as nullable.

[^1]: The entire work effort of this PR was trying to verify this
statement.


[APG-131]:
https://hasurahq.atlassian.net/browse/APG-131?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

changelog.md

@@ -9,6 +9,10 @@
 
 ### Changed
 
+- Aggregate functions (except COUNT) are now all marked as having nullable
+  return types, as they will return null for empty row sets.
+  ([#498])(https://github.com/hasura/ndc-postgres/pull/498))
+
 ### Fixed
 
 - Rows and aggregates parts of the query should operate on the same query parameters (where, order by, limit and offset).

crates/connectors/ndc-postgres/src/schema.rs

@@ -38,8 +38,14 @@ pub fn get_schema(
                         (
                             function_name.clone(),
                             models::AggregateFunctionDefinition {
-                                result_type: models::Type::Named {
-                                    name: function_definition.return_type.0.clone(),
+                                result_type: models::Type::Nullable {
+                                    // It turns out that all aggregates defined for postgres
+                                    // (_except_ `COUNT`) will return `NULL` for an empty row set.
+                                    // Thus, we mark all aggregates as having a nullable return
+                                    // type.
+                                    underlying_type: Box::new(models::Type::Named {
+                                        name: function_definition.return_type.0.clone(),
+                                    }),
                                 },
                             },
                         )