fix: Heikin-Ashi docs

[DaveSkender]

During evaluation of #1160 I found an error in our Heikin-Ashi documentation. The first set of values is no longer null as there is a reasonable starting result based on the initial candle value.

To do:

• remove reference that indicates the first period would be null
• refactor code to only evaluate first period once. This should provide a slightly better performance.

[github-actions]

✔️ Tests 718 / 718 - passed in 6.4s
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✏️ updated for commit 70e1659

[DaveSkender]

This micro-optimization for initialization improved the performance test by 0.001us No difference, really. Still, seems like a better practices so each iterative loop isn't re-evaluating condition.