fix: preserve whitespace around empty JSX expressions

[raisulchowdhury]

Summary

• Preserve an empty JSX expression when removing it would change whitespace across a multiline JSX boundary.
• Keep the existing removal behavior for inline empty expressions where no rendered whitespace is lost.
• Add the reported regression and a safe inline control case.

Fixes #3995.

Validation

• npx mocha tests/lib/rules/jsx-curly-brace-presence.js --grep 'The braces' (2 passing)
• Direct ESLint Linter verification of the reported case and inline control
• npx eslint lib/rules/jsx-curly-brace-presence.js tests/lib/rules/jsx-curly-brace-presence.js (0 errors; 7 pre-existing warnings in the test file)
• git diff --check

The full legacy parser matrix is not a reliable signal in this environment because its TypeScript 3.9 parser stack is incompatible with the installed Node 25 runtime; the focused rule tests and direct behavior checks pass.

Assisted-by: OpenAI Codex. The contributor reviewed the implementation and validation before publication.

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[haimuhaimu]

The whitespace-preservation guard misses the equivalent empty no-substitution template literal case.

I reproduced this on head 178859e73e3c81165b73696a363e00fa9f314ed6 with ESLint 9.39.5 and verifyAndFix:

<span>
  <span>The braces</span>
  {``} matter here.
</span>

The rule still reports/fixes the empty template expression and produces:

<span>
  <span>The braces</span>
   matter here.
</span>

That is the same whitespace-sensitive output shape as #3995. lintUnnecessaryCurly explicitly handles a TemplateLiteral with zero expressions and the fixer replaces it with its empty cooked value, but isEmptyStringExpression only checks child.expression.value === ""; a TemplateLiteral has no value, so the new bailout never runs.

Could isEmptyStringExpression also recognize a zero-expression template whose sole quasi has an empty cooked/raw value? A multiline valid regression using {``} plus the existing safe inline invalid/control case would cover both branches.

[haimuhaimu]

Verified the follow-up on d0a5dedeff02e86ba09b089e877e192c4f292a82 with ESLint 9.39.4. The multiline empty-template reproducer now remains unchanged with no diagnostic, the inline control still fixes to plain text, and the existing empty-string multiline case remains protected. The added regression file is included by the project unit-test glob, and all four reported checks pass. My previous concern is resolved; thanks for covering both branches.

[haimuhaimu]

Re-verified the latest head 3e26b04c39cde640062ec5719434abf488f2cf74, including the new no-children guard added after my previous approval. With ESLint 9.39.4, the JSX attribute control now fixes <App prop={foo} /> to <App prop="foo" /> without throwing; the multiline empty-template and empty-string cases remain protected, and the inline template control still fixes normally. All four reported checks pass. This approval covers the current head.