[Hanabi9248]

Fixes #420.

An indexed source map containing sourcesContent: ["a\nb\nc\n", ""] treats the second source as absent: sourceContentFor() discards the valid empty string and either throws or returns null. SourceMapGenerator.fromSourceMap() consequently throws instead of preserving the empty file.

Accept content when it is non-null rather than truthy in IndexedSourceMapConsumer.sourceContentFor. Keep searching sections for null/undefined and retain the existing unknown-source exception/nullOnMissing behavior. No generator or missing-content policy changes are needed.

Validation on exact master 8c668f3dd3916e96d06131dec97c78a3e62bd0f3, Node.js v24.19.0:

• Eight new methods use real Basic/Indexed SourceMapConsumer construction, real SourceMapGenerator conversion and finally/destroy cleanup. Original source: five failures, three controls pass. Proposed source: 8/8 pass. Cases include the original issue input, later-section lookup, nested indexed maps, empty versus null content, nonempty content and missing-source behavior.
• Existing consumer suite: 64/64 pass. Existing generator suite: 29/29 pass. These are separate suites, not a combined rerun.
• Official support sources and the actual WASM blob 7fc7d86ea2eeaa49762fc0dbbd1c3086c0447ee3 are bound to this base; no fake consumers, private-field replacement or WASM stubs.
• Validation ran serially under one CPU, 768 MiB for the whole job, 16 processes, private TMP and BLAS threads set to one; peak job memory across these runs was 36,708,352 bytes. Production-source whitespace check passes.

Browser integration, other Node.js versions and the full repository suite were not run; no dependencies or toolchains were installed. The nullOnMissing policy changes proposed in #391 are outside this fix.

AI assistance was used for investigation, implementation and validation.