typings: impact property of Result and NodeResult should be optional

[dbjorge]

Expectation: https://www.deque.com/axe/axe-for-web/documentation/api-documentation/#result-arrays documents that impact can be null in for results or nodes that correspond to passing results; the corresponding impact properties in axe.d.ts of the Result and NodeResults types should accept null values, accordingly.

Actual: axe.d.ts specifies those impact properties as non-optional

Motivation: We see type-checking errors in our consuming project.

axe-core version: 3.2.2
axe-webdriver, extension or other integration version: n/a

Browser and Assistive Technology versions

For Tooling issues:
- Node version: current LTS (10.15.3)
- Typescript version: current stable (3.4.1)
- Platform:  n/a (Windows)