fix: accept null/empty-string in fragments and harden test coverage

[voxpelli]

Summary

• html tagged template correctly normalizes falsy interpolation values (null, undefined, false) to '' via _checkHtmlResult, but multi-root fragments kept those empty strings in the returned array
• render() rejects '' via its own falsy guard (if (!item) throw), causing html${element}${null}${null}`` to throw at render time
• Fix: filter '' from the array output in html() so only meaningful renderable values reach the render paths — fixes both sync and async paths automatically
• Type-narrow render()/renderSync() to accept empty strings as valid HtmlMethodResult

Code review follow-ups

• Align /* c8 ignore next */ comment style with -- exhaustiveness guard annotation across render.js
• Replace weak assert.equal(result.length, 1) with assert.deepEqual(result, [element]) in array filtering test
• Add Promise.resolve(null) fragment test covering the async path (the stated PR motivation)
• Add renderToStringSync('') test mirroring the async render('') test
• Add symbol rejection type test for HtmlTemplateValue

Test plan

• End-to-end async tests for null, false, undefined, and Promise.resolve(null) fragment interpolations
• End-to-end sync tests for null, false, undefined, and empty string fragment interpolations
• Array shape test verifying empty strings are filtered with content assertion
• false and symbol type assertions for HtmlTemplateValue (typetests/index.test.ts)
• Full test gauntlet passes: tsc, eslint, knip, type-coverage ≥99%, tstyche (TS 5.8 + 5.9), 196 runtime tests

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes a rendering bug where multi-root html``...`` results could contain '' entries (from null/undefined/false interpolations), which then caused render() / renderToStringSync() to throw due to their non-falsy guards.

Changes:

• Filter '' values out of multi-root array results returned by html() to prevent render-time failures.
• Introduce/extend template typing via HtmlTemplateValue and add related type-level regression coverage.
• Add runtime tests covering fragment interpolations for null/false/undefined in both async and sync render paths.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lib/htm.js	Filters empty-string entries from multi-root html() arrays; updates value typing for the internal htm binding.
lib/element-types.d.ts	Adds HtmlTemplateValue and adjusts ElementPropsValue Set/Map typing.
test/render-complex.spec.js	Adds async end-to-end tests for fragment interpolations (null/false/undefined) and removes now-unnecessary TS ignores.
test/sync-render.spec.js	Adds sync end-to-end tests for fragment interpolations (null/false).
test/html.spec.js	Adds an array-shape test asserting '' values are filtered from multi-root results; tweaks TS ignore comment.
test/async-render.spec.js	Removes TS ignore comments now covered by updated typing.
typetests/index.test.ts	Adds type regression tests for HtmlTemplateValue composition and Set/Map assignability.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

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