POWER Needs to Accept NULL Args

[oleibman]

Investigating issue #1622, I found that its extremely complicated formula, which had been leading to an incorrect result, now led to an exception. I am able to fix the exception; unfortunately, I am no closer to resolving the original issue. So I'll apply the baby step while continuing to investigate. Function POWER had changed from untyped args to a defined set of types. The set of types was determined according to the doc block, but that was incomplete - it had neglected to include null and bool. This PR corrects the function prototype and the doc block, and adds the missing tests for those conditions.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.