When the parsed files have no associated filenames (eg: a string or array of strings was passed to the `minify` function), the `sources` array of the generated sourcemap must only contain null values.
Currently, it contains the stringified indexes of the array. For example, passing a string to `minify` results in a `sources` array of `["0"]`, which breaks the sourcemap.
Also, this PR adds the `sourceMap.json` option. When explicitly set to `false`, the sourcemap is *not* stringified.
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