Omit optional parameter in the variables field of a request when the parameter value is null

^{Originally posted by **CalvinQuark** November 7, 2023}
There is a compatibility issue when using a ChilliCream StrawberryShake generated client to fetch data from an Azure Data API Builder CLI-generated GraphQL endpoint.

If a query is defined with an optional `$filter` parameter such as:

```gql
query Todos(
$filter: TodoFilterInput
) {
todos(
filter: $filter
) {
items {
Id
Title
DueOn
}
}
}
```

When a `null` value is passed to `filter` parameter via a ChilliCream StrawberryShake-generated client query's `.ExecuteAsync(filter: null)` method, the request does not include the `"filter": null` node in the `variables` section:

```json
{
"id": "ec304833fcb193a31fdd994a6c503e21",
"query": "query Todos($filter: TodoFilterInput) { todos(filter: $filter) { __typename items { __typename Id Title DueOn } } }",
"operationName": "Todos",
"variables": {}
}
```

Unfortunately, the Azure Data API Builder CLI's GraphQL endpoint responds to this request with:

```json
{
"errors": [
{
"message": "The variable with the name `filter` does not exist."
}
]
}
```

unless the `null`-valued `filter` variable is specifically included in the request payload. I.e., this works:

```json
{
"id": "ec304833fcb193a31fdd994a6c503e21",
"query": "query Todos($filter: TodoFilterInput) { todos(filter: $filter) { __typename items { __typename Id Title DueOn } } }",
"operationName": "Todos",
"variables": {
"filter": null
}
}
```

I haven't discovered a workaround for this issue either on the client side with StrawberryShake or on the server side with Azure Data API Builder. Is there a particular reason why it is necessary to include the unused null `filter` variable? If so, this seems unintuitive since the query parameter is declared as nullable `$filter: TodoFilterInput`. I.e., it's not suffixed with the required operator (`!`).