fix: HX-Trigger with null event detail no longer throws

[Lluczel]

Description

An HX-Trigger response header whose event detail is null, e.g. HX-Trigger: {"myEvent": null}, throws TypeError: Cannot read properties of null (reading 'cancelled'), and the event is never dispatched.

__handleTriggerHeader decides whether to pass the detail through as-is with typeof detail === 'object'. Since typeof null === 'object', null is passed straight to trigger(). Its detail = {} default only applies to undefined, so !detail.cancelled throws.

This change excludes null from the pass-through branch, so it is wrapped like any other non-object value ({value: null}), the same way {"myEvent": 42} becomes {value: 42}. Object details and the comma-separated string form are unchanged.

The issue suggests a guard in trigger() instead. I kept the fix where the header is parsed for two reasons: a header-supplied null then follows the existing rule for non-object details (handlers read event.detail.value), and the public trigger() API stays untouched. Happy to add a guard there as well if you'd prefer.

HX-Trigger: {"myEvent": null}

Corresponding issue: #4135

Closes #4135

Testing

• Added wraps null detail in value property to test/tests/unit/__handleTriggerHeader.js. It fails on four-dev with the TypeError above and passes with this change.
• npm test (Chromium): 1781 passed, 0 failed, 4 skipped.
• python3 src/scripts/content/check.py: OK.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes) (targeting four-dev, since this is an htmx 4 source fix)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded