Allow Nested FilterInputType to have `isNull` check

Product

Hot Chocolate

Is your feature request related to a problem?

Currently when creating a FilterInputType for a type (example below):

public class AccountFilterInputType : FilterInputType<Account>
{
    protected override void Configure(IFilterInputTypeDescriptor<Account> descriptor)
    {
        descriptor.BindFieldsExplicitly();
        descriptor.Field(a => a.Transactions!.OrderByDescending(t => t.CreateTime).FirstOrDefault())
            .Name("latestTransaction")
            .Type<TransactionFilterInputType>();
    }
}

It's not possible to check if the field value is null.
This is useful when querying for a scenario where you want: Accounts that have no transactions OR accounts that has Count() > 0 transactions.

The current workaround is to create a "hasX" field. Example:

descriptor.Field(a => a.Transactions!.Any())
            .Name("hasTransactions");

The solution you'd like

I'd like to be able to instead of having to create a separate "hasX" field to be able to check if the value is null or not. Example:

query SearchAcounts($workspaceId: UUID!, $userId: UUID!) {
  account(workspaceId: $workspaceId) {
    searchAccounts(
      where: {
        latestTransaction: { 
          isNull: {
             eq: false
          }
        }
      }
    ) {
      ...
    }
  }
}

Start by tracing how FilterInputType and IFilterInputTypeDescriptor configure the nested latestTransaction field in the AccountFilterInputType example, then compare that path with existing isNull filter handling. Done means nested filter fields can accept the shown isNull: { eq: false } condition without requiring a separate hasTransactions field, with coverage for the null and non-null cases.