fix: avoid rendering literal "undefined" for unset src/alt/value attrs

CLAUDE.md

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+# Tradeshift Elements — Repo Notes for Claude
+
+## Build all packages together, never one in isolation
+
+Every package's `render()` imports `html`/directives that must come from the **same**
+lit-html instance across `packages/core` and every `packages/components/*` package.
+`npm run build` (root) builds everything in one pass and keeps this consistent.
+
+Building a single package on its own (e.g. `cd packages/components/app-icon && npm run
+build`, or copying just one package's `lib/` output somewhere) can leave it bundled
+against a different lit-html instance than its siblings. Directives like `ifDefined`
+then silently stop being recognized (they get treated as a plain, unrecognized value
+instead of "no value"), with no error — the symptom looks like the property never
+received its value. If you see that symptom after a build, suspect this first.
+
+## `check-deps` (depcheck) doesn't account for `test/` directories
+
+The pre-commit hook runs `check-deps` (`lerna exec depcheck --ignore-patterns=lib,types`).
+That ignore list doesn't include `test/`, so a devDependency used only in
+`packages/components/*/test/*.test.js` (e.g. `@open-wc/testing`) gets flagged as
+"missing" even though it's a legitimate root-level devDependency for tests. This is a
+pre-existing gap, not a real problem — safe to bypass with `--no-verify` for this
+specific failure, but see the rule below.
+
+## Disclose `--no-verify`
+
+If a commit uses `--no-verify`, say so explicitly in the commit message or PR
+description: which hook step failed, why it's safe to skip, and whether it's tracked as
+follow-up work. Don't just skip silently.
+
+## No unit test framework existed before this repo note was added
+
+`npm test` is `npm run happo` — Happo.io visual-regression snapshots only, no
+assertion-based tests. `@web/test-runner` + `@web/test-runner-playwright` +
+`@open-wc/testing` were added (see `web-test-runner.config.mjs`, `npm run test:unit`) as
+the pattern for component-level regression tests going forward. Test files import from
+each component's `../lib/*.esm.js` (built output), not `../src/*.js` directly — `src/`
+files import raw `.css` and other assets that only become valid JS through the Rollup
+build pipeline; `@web/test-runner`'s dev server doesn't transform those the way Rollup
+does. Run `npm run build` before `npm run test:unit` (wired up via `pretest:unit`).
+
+Avoid jsdom for testing these components: this repo's Babel/Rollup output for some
+lit-element class hierarchies (native-class extension via `Reflect.construct`) hits real
+jsdom incompatibilities that don't reproduce in an actual browser. `@web/test-runner`
+with the Playwright launcher runs tests in real Chromium instead.