Handle null values in primitive types for Variants and TValue types

[wlandgraf (Wagner Landgraf)]

In schema scalar serializers, return null with values are variant (NULL) or TValue (Empty).

Reference:

Nullable Float/Int fields

In GraphQL all scalar types are nullable by default. In GraphQL for Delphi, when using a schema type with a nullable Int or Float field, there seems to be no built-in way to make the resolver function return null. I modified the TSchemaFloatType.Serialize function to explicitly check for a null variant type. The updated code snippet is provided below: function TSchemaFloatType.Serialize(const Value: TValue): TValue; begin if Value.IsEmpty or ((Value.Kind = tkVariant) and (Value.AsVarian…