Editor incorrectly evaluates a curve set

[LeXXik]

Under some conditions, the Editor creates a Curve instead of a CurveSet, which breaks the app.

Forum: https://forum.playcanvas.com/t/weird-issues-with-the-curve-attribute-in-editor/38029/5
Repro: https://playcanvas.com/project/1269201/overview/curve-set-issue

Steps:

1. Launch app, and click Play in UI.
2. Depending on the current state in Editor, the box will animate or not.
3. Select the box in Editor and add a new array element to the motion script attribute (keep defaults).
4. Launch app, and click Play in UI screen. The box will fail to animate, as the Editor creates a Curve, instead of CurveSet for the position field.