Fix type of chartInstance emitter

[Nosfistis]

Describe the bug

The (chartInstance) EventEmitter does not mention that it might emit a null value. This happens on component destroy, where the emitter emits a null value.

Expected behavior

Either the emitter should not emit a null value, or the (chartInstance) emitter should include it in its type declaration.

[karolkolodziej]

Hi @Nosfistis!
You are right! Thank you for reporting this!

[ivankokan]

@karolkolodziej, please update the docs as well; it mentions only the creation, not the destruction.

highcharts-angular/README.md

Line 234 in 65ff141

* **component output `chartInstance`** - emitted after chart is created (see [demo app](#demo-app) and `logChartInstance` function)

[PowerKiKi]

Documentation is updated in #402

[ivankokan]

@PowerKiKi Thanks!