Make null/undefined a noop for addFrames

[rreusser]

This PR makes passing either null or undefined to Plotly.addFrames a no-op. I didn't initially expect this would be a meaningful case, but this allows all plots to simply call

Plotly.plot(gd, x.data, x.layout, x.config).then(function () {
  return Plotly.addFrames(gd, x.frames))
});

without worrying about whether addFrames needs to be called. Ideally in v2.0 frames will be part of the initial plot call, but until then, at least you can call addFrames and let the method sort out whether it's a no-op or not.

[etpinard]

Ideally in v2.0 frames will be part of the initial plot

For the record, we could also add a new call signature to Plotly.plot e.g:

Plotly.plot(graphDiv, {
  data: [],
  layout: {},
  config: {},
  frames: []
});

[etpinard]

looking good 💃