highcharts: titles can be null

[michaelabon]

From their documentation:

https://api.highcharts.com/highcharts/title.text
https://api.highcharts.com/highcharts/yAxis.title.text

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes:
  • https://api.highcharts.com/highcharts/title.text
  • https://api.highcharts.com/highcharts/yAxis.title.text
• Increase the version number in the header if appropriate.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.

[dt-bot]

types/highcharts/index.d.ts

to authors (@damianog @baltie @AlbertOzimek @hanssens). Could you review this PR?
👍 or 👎?

[typescript-bot]

@MKenyon Please fix the failures indicated in the Travis CI log.

[RyanCavanaugh]

@MKenyon Please fix the failures indicated in the Travis CI log.

[michaelabon]

@RyanCavanaugh Happy to do so, but there appear to be errors with botvs. Should I wait until those are fixed?

[michaelabon]

Okay, I found that #21125 would fix the lint errors with the botvs types, but the author @kevin-greene-ck closed the PR. I guess I'll resubmit the PR and then rebase this PR on top of it.

[michaelabon]

So, #20933 has another similar fix to #21125 (and #21163). In #20933, @atd-schubert explains that there is a new linting-rule in tslint (palantir/tslint#2754). @atd-schubert also explains that they made an identical change in #20932.

Once #20932 is merged, these issues with botvs should be resolved. Then the highcharts change in this PR can go through. 🤕

[atd-schubert]

Go for it 🤓...

[michaelabon]

Hello @damianog @baltie @AlbertOzimek @hanssens,

Would you be able to review this PR, please?

(Thanks @atd-schubert!)

[baltie]

LGTM 👍 Thanks @MKenyon!

[RyanCavanaugh]

Approved by a listed owner. PR ready to merge pending express review by a maintainer.