[zhanghao0806]

Description

Treat JSON envelope attributes whose value is null as omitted before version detection, time parsing, and event construction. The core JSON reader previously passed them to validation (or isoparse for time), rejecting JSON representations permitted by the specification.

Related to the JSON decoding distinction discussed in #199. This does not change Python object equality or in-memory attribute validation.

Specification: https://github.com/cloudevents/spec/blob/v1.0.2/cloudevents/formats/json-format.md#22-type-system-mapping

Validation

• Before the fix: 11 new regression cases failed.
• Full pytest -q: 886 passed, 18 xfailed.
• Ruff lint/format and MyPy on both changed files: passed.
• Covers optional context attributes and extension attributes, explicit and auto-detected factories, a custom factory, null payloads, and nested payload null/false/zero values.

Only top-level null values are removed; payload contents are preserved. Prepared with AI assistance; commits are signed off.