When creating chunks, don't write empty partials array or ttsInstructions object to JSON

[matbind]

In chunk.ts empty partials arrays or ttsInstructions objects are written to the resulting JSON file, e.g.:

{ time: 0, partials: [ [Object] ], ttsInstructions: {} }

and

{
  time: 0,
  partials: [],
  ttsInstructions: { '0': [Object], '1': [Object] }
}

This is not ideal, and should be an easy way to save on file size.