[Feature] Handle null sections in indexed sourcemap

[EmmaYuan1015]

• Prepare an indexed sourcemap where one section has map: null.
• Call SourceMapUtil.updateSourceMapFile with this sourcemap content.
• In some cases flattening fails and updateSourceMapFile returns the original sourcemap body from catch path.

Expected behavior
Null sections should be normalized safely.
Indexed sourcemap should still be flattenable or handled by a robust fallback.
updateSourceMapFile should return a transformed sourcemap with updated file/sourceRoot/sources.