[palette_generator] Migrate to null safety

[mzdm]

Related tracking issue flutter/flutter#75567

[stuartmorgan]

I did a quick initial pass, but @gspencergoog should do the full review.

[stuartmorgan]

Everything I saw has been addressed; @gspencergoog should definitely do the final review here though since he knows the API.

[gspencergoog]

Looking at it today.

[mzdm]

after this revision all tests except one pass
(PaletteGenerator Filters work test)

dart:core                                                _AssertionError._throwNew
package:palette_generator/palette_generator.dart 74:16   new PaletteGenerator.fromColors
package:palette_generator/palette_generator.dart 124:29  PaletteGenerator.fromImage

'package:palette_generator/palette_generator.dart': Failed assertion: line 74 pos 16: 'paletteColors.isNotEmpty': is not true.

[gspencergoog]

Sigh. Conceptually, it should be possible to enforce that paletteColors not be empty, but if the image contains all transparent pixels, or all the colors that exist in the image have been ignored by the caller, then it will indeed be empty. We could throw if one of those conditions happens, but I think it's better to just allow the list to be empty, which will result in a nullable dominantColor, which makes the most sense if there are no candidate colors. Thanks for trying that out anyhow.

[gspencergoog]