Handle Nullable types

[m-wild]

See #16

Also, we should probably be handling Nullable differently to other generics...
Json Schema allows for { "type": "null" }, see: https://json-schema.org/understanding-json-schema/reference/null.html
Which means we could detect System.Nullable and set the property as { "type": [ "T", "null" ] }

[neoaisac]

Thinking of looking into this for Hacktoberfest. Any pointers?

[m-wild]

We generate a Document which contains Messages, each message has a Payload.

https://github.com/tehmantra/saunter/blob/279f51fce99847009b43badb522428638e3b6688/src/Saunter/Generation/DocumentGenerator.cs#L253

This ISchema is in JSON-Schema format.

When we set the Schema.Type property (which is currently a string) by reflecting types in the SchemaGenerator.

https://github.com/tehmantra/saunter/blob/279f51fce99847009b43badb522428638e3b6688/src/Saunter/Generation/SchemaGeneration/SchemaGenerator.cs#L31:L49

An example of a simple schema representing a non-nullable int:

{
    "type": "integer"
}

An example representing a nullable int:

{
    "type": ["integer", "null"]
}

We need to make Schema.Type render as either a string "T", or if the type is nullable, an array containing [ "T", "null" ].

We can optionally/additionally support the OpenAPI spec which defines an additional property nullable: bool
Example of nullable int in OpenAPI:

{
    "type": "integer",
    "nullable": true
}

[m-wild]

This is implemented using a [Flags] enum in Newtonsoft.Json.Schema
https://github.com/JamesNK/Newtonsoft.Json.Schema/blob/master/Src/Newtonsoft.Json.Schema/JSchemaType.cs

Same as NJsonSchema
https://github.com/RicoSuter/NJsonSchema/blob/master/src/NJsonSchema/JsonObjectType.cs

I suppose this is the easiest way to be able to set this as one or more value.

// for non-nullable primitives
Type = JsonSchemaType.Integer;

// for nullable-primitives
Type = JsonSchemaType.Integer | JsonSchemaType.Null;

This also allows for setting the nullable type separate from the underlying type;

var type = JsonObjectType.Integer;

//... elsewhere
if (nullable) {
    type = type | JsonObjectType.Null;
}

[m-wild]

Closing in favor of #60