[lexical] Bug Fix: don't leave empty class/style attributes in the editor DOM

[LeSingh1]

classList.remove() and style.setProperty(prop, '') keep the attribute in place with an empty value, so clearing the last theme class or the last inline declaration renders <span class=""> / <p style="">.

Repro: p.setFormat('center') then p.setFormat('') gives <p style="">; toggling selection.formatText('underline') twice with a theme.text.underline class gives <span class="">.

Adds an internal removeEmptyDOMAttribute(dom, 'class' | 'style'), which drops the attribute when getAttribute returns '', and calls it from the three reconcile paths that can drop the final token: element format (text-align), element indent (padding-inline-start plus the indent theme class), and TextNode.updateDOM (theme classes and __style). It is not exported from index.ts, so there is no public API or flow change.

Four unit tests, all failing on main with the exact style="" / class="" output.

Three e2e files had recorded the leftover attribute and are updated — ClearFormatting (5), Indentation (6), KeyboardShortcuts (1). Their purpose is unchanged; only the stale attribute is gone, and I confirmed in Chromium beforehand that the received HTML differed from the expectation by exactly style="".

DraggableBlock.spec.mjs keeps its four style="" assertions on purpose: that one comes from LexicalDraggableBlockPlugin writing element.style.transform directly and restoring it, not from the reconciler. Same cosmetic class, plugin-owned DOM, worth its own fix.

Noting @acywatson's "we could probably change this, but I'm not sure it's a bug" from the thread — happy to close this if the answer is still that it isn't worth the churn.

Fixes #5307

[vercel]

@LeSingh1 is attempting to deploy a commit to the Meta Open Source Team on Vercel.

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[LeSingh1]

Consolidated into #9058 with the other PRs that share this defect, per @etrepum's note on #9027 and @mayrang's on #9035. Same fix and same tests, one review.