Fix JavaScript "Error: addFrames failure" in offline html export

[jonmmease]

Fix for "Problem 3" in #1147.

Since version 3.0, stand-alone html files/divs generated by plotly.offline.plot resulted in a harmless JavaScript error on load if no Frames were defined.

Uncaught (in promise) Error: addFrames failure: frameList must be an Array of frame definitions[object Object]
    at Object.r.addFrames (pca:73)
    at pca:79

The problem was that the no-frames case was being represented by an empty dict rather than an empty list. Also, the frames logic should not have been output at all for the empty frames case.

cc: @CiaranWelsh