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.

How was this tested?

test_include_null_cli_option (parametrized over no flag, --include-null, and
--no-include-null): an optional scalar slot is ["string", "null"] by default and
"string" under --no-include-null; an optional multivalued slot is
["array", "null"] vs "array"; a required slot is unaffected in all cases.

Areas of uncertainty

• The default (--include-null) preserves current upstream behaviour; whether upstream would prefer flipping the default (a breaking change for consumers relying on explicit null) is a maintainer decision.

Checklist

• My code follows the contributor guidelines
• I have added tests that prove my fix/feature works
• Existing tests pass locally with my changes

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