Fix JS syntax error when embedding an empty file.

If the file_packer is asked to embed an empty file, it will currently generate invalid javascript that looks like this:

  Module['FS_createDataFile']('/some/path', 'filename', , true, true)

This PR special-cased the representation of empty files to generate the correct syntax:

  Module['FS_createDataFile']('/some/path', 'filename', [], true, true)