Add null safety support about flutter_home_indicator

[azeek]

I hope this problem is solved quickly.

[light-speak]

I hope this problem is solved quickly.

[alexmarkley]

Thanks so much for this package! This package is a huge help to me and my app definitely depends on it.

I agree that null safety support would be very helpful.

[nohli]

You can use my migrated fork for now:

  home_indicator:
    git:
      url: https://github.com/nohli/flutter_home_indicator
      ref: migrate-to-nnbd

Also @lynn could merge the PR and publish a new stable version to pub.dev.