Error creating a SourceMapGenerator from a SourceMapConsumer of an index map (part 2)

;(async () => {
  let mozilla = require('source-map')
  let map;

  map = {
    version: 3,
    sections: [
      {
        offset: { line: 0, column: 0 },
        map: {
          version: 3,
          sources: ['123', '456'],
          sourcesContent: ['a\nb\nc\n', ''],
          mappings: 'AAAA;AACA;AACA;'
        }
      }
    ]
  }

  // this map works for some reason
  /*map = {
    version: 3,
    sources: ['123', '456'],
    sourcesContent: ['a\nb\nc\n', ''],
    mappings: 'AAAA;AACA;AACA;'
  }*/

  let consumer = await new mozilla.SourceMapConsumer(map)
  consumer.sourceRoot = null; // https://github.com/mozilla/source-map/issues/345
  let generator = mozilla.SourceMapGenerator.fromSourceMap(consumer)
  console.log(generator.toJSON())

})().catch(console.log)

This errors out:

Error: "456" is not in the SourceMap.
    at IndexedSourceMapConsumer.sourceContentFor (/tmp/node_modules/source-map/lib/source-map-consumer.js:840:11)
    at /tmp/node_modules/source-map/lib/source-map-generator.js:82:42
    at Array.forEach (<anonymous>)
    at Function.fromSourceMap (/tmp/node_modules/source-map/lib/source-map-generator.js:72:32)
    at /tmp/123/test.js:29:46

The issue is in here somewhere:

https://github.com/mozilla/source-map/blob/7a263408de1b60e60b6220ca3e36d9b3e4aa047d/lib/source-map-consumer.js#L880

Content is empty - '' - but it is still in the sourcemap. I believe it should be if (content == null) { check, i.e. null or undefined, letting empty string pass along.

this may be related to https://github.com/mozilla/source-map/issues/345, so tagging it just in case

Start in lib/source-map-consumer.js around the sourceContentFor implementation and trace how lib/source-map-generator.js calls it from fromSourceMap. Run the provided index-map reproducer and confirm that an empty sourcesContent entry is preserved rather than treated as missing, without changing the behavior for null or undefined content.