fix(packages/core): allow empty OutputOptions.format

[hyf0]

Description

Test Plan

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[hyf0]

Current dependencies on/for this PR:

• main
  • PR feat: generate default export for cjs entry #219
    • PR fix: should rewrite export default import(...) correctly #220
      • PR chore: refine @rolldown/core package #221
        • PR fix(packages/core): make outputOptions optional #223
          • PR fix(packages/core): allow empty OutputOptions.format #224 👈

This stack of pull requests is managed by Graphite.

[hyf0]

Merge activity

• Nov 11, 1:40 AM: @hyf0 started a stack merge that includes this pull request via Graphite.
• Nov 11, 1:43 AM: Graphite rebased this pull request as part of a merge.
• Nov 11, 1:44 AM: @hyf0 merged this pull request with Graphite.