More elegantly handle error when attempting to animate single-sample trajectory

[fedarko]

From biocore/empress#319:

Problem 1

When certain fields are selected as the trajectory / gradient in Emperor, I see an Uncaught TypeError: Cannot read property 'value' of undefined error in the console:

This is an Emperor feature/bug if you see the traceback this originates from the animations-controller.js. This happens when only single-sample trajectories are selected so there's nothing to animate. This should be better handled on the Emperor side.

Documenting this here for future reference.