Stricter return value check on LGraphCanvas.getExtraMenuOptions

[huchenlei]

In javascript !![] evals to true, so the old check would always pass if the callback obeys the type signature, which can cause an extra null item being added (separator). This PR fixes the issue which makes empty array return value generate a separator.

getExtraMenuOptions?(
  this: LGraphNode,
  canvas: LGraphCanvas,
  options: IContextMenuValue[],
): IContextMenuValue[]

[webfiltered]

Comment for history only: return type was implemented without consideration for explicit | undefined req. for optional return - missed due to strict mode disabled.