TypeError when creating a SourceMapGenerator from a SourceMapConsumer of an index map

When calling SourceMapGenerator.fromSourceMap with a SourceMapConsumer for an index map, the following error is thrown:

TypeError: Cannot read property 'replace' of undefined
    at Object.relative (C:\dev\scratch\source-map-test\node_modules\source-map\lib\util.js:255:17)
    at C:\dev\scratch\source-map-test\node_modules\source-map\lib\source-map-generator.js:75:31
    at Array.forEach (<anonymous>)
    at Function.fromSourceMap (C:\dev\scratch\source-map-test\node_modules\source-map\lib\source-map-generator.js:72:32)
    at test (C:\dev\scratch\source-map-test\test.js:17:52)
    at <anonymous>

test.js

const sourcemap = require("source-map");
async function test() {
    const map = { 
        file: "generated.js",
        version: 3,
        sections: [
            { offset: { column: 0, line: 0 }, map: {
                file: "part.js",
                version: 3,
                sources: ["source.js"],
                mappings: "",
                names: []
            } }
        ]
    };
    const consumer = await new sourcemap.SourceMapConsumer(map);
    const generator = sourcemap.SourceMapGenerator.fromSourceMap(consumer); // TypeError
}

The error seems to be due to a strict inequality (!==) check on sourceRoot whereas all other checks against sourceRoot use loose inequality (!=).

Start with lib/source-map-generator.js around fromSourceMap and lib/util.js around relative, then reproduce the failure using the test.js example in the issue. Add a regression test for converting an index-map SourceMapConsumer, and consider the existing sourceRoot comparisons. Done means conversion no longer throws and the relevant tests pass.