feat(jsonschemagen): add --include-null/--no-include-null CLI option

[jdsika]

Summary

Expose the existing JsonSchemaGenerator.include_null field (default True) on the
gen-json-schema CLI as --include-null/--no-include-null. No generator logic
changes — the option is forwarded to the existing field.

Motivation (standards)

include_null decides whether an optional slot is rendered as ["<type>", "null"]
(accepts an explicit JSON null) or as the bare "<type>".

• JSON Schema Validation, draft 2019-09, §6.1.1 (type) — the allowed type
  tokens are the six primitive types (null, boolean, object, array, number,
  string). null is a value type, so a type array is the standard mechanism for
  declaring whether an explicit null is an acceptable value.
• JSON Schema Validation, draft 2019-09, §6.5.3 (required) — presence/absence
  of a property is a separate concern from type.
• RFC 8259, §3 (Values) — null is a literal JSON value, distinct from an absent
  member (null ≠ absent).

Because required (§6.5.3) already expresses optionality, adding "null" to type
to model "optional" overloads the value-type keyword. --no-include-null yields the
idiomatic rendering of a non-nullable optional ({"type": "string"} + omission from
required), required for strict structural parity with normative reference schemas
that declare a bare type and forbid null.

Changes

• One click option on gen-json-schema, forwarded to the existing include_null
  generator field.

Backward compatibility

Default remains include_null=True; output is byte-identical unless
--no-include-null is passed.

Test

test_no_include_null_omits_null_from_optional_slots: an optional slot is
["string", "null"] by default and "string" under include_null=False; required
slots are unaffected.