[BUG] Motion 11 no longer support label as first step

[bakura10]

Hi,

While upgrading from Motion 10 to 11, my code broke when I was specifying a named label as the first step:

animate([
  "start",
  [fromPicture, { opacity: [1, 0] }, { duration: 0.3, at: "start" }],
  [toPicture, { opacity: [0, 1] }, { duration: 0.3, at: 'start' }]
]);

Motion 11 now throws an error when a label is set at the first step of a sequence.