Chore: adjust pie sector key value

[imagineLife]

Description

Updates the sector (pie "slice") rendering logic to render null at the "beginning" of an animation.
This allows for more explicit key values of sectors once the sectors have starting+ending angles.
The no-array-index-key seems to only be appearing in pie chart sectors when (1) animation is enabled (2) the first rendering of each slice where each slice has "nothing" to render that can uniquely identify each sector. Once the slices "have size" in the animation, the no-array-index-key stops appearing.

I have a hunch that this starting-animation key detail may be present in other chart types, too.

Related Issue

3273

Motivation and Context

set more explicit key value in a pie chart.

How Has This Been Tested?

• pie demos work without key error: Pie, PieChart, ResponsiveContainer, all "manual" testing
• unit tests are updated & pass

Types of changes

• technical detail

Checklist:

• My code follows the code style of this project.
• I have added tests to cover my changes
  • assert 1-slice pie renders with animation
  • update 6-slice pie to include animation & v attribute seen in demo data code
• All new and existing tests passed

[ckifer]

some questions

[ckifer]

Let's merge and monitor again, looks good to me though