Add null handling to json serialization

[manzanotti]

The move to System.Text.Json from Json.Net for v4 had the unfortunate side-effect of removing null handling, causing issue #21 to re-appear.

This PR adds it back in.

[ctolkien]

Thanks for this.

This is probably something we should add unit tests for at some point.

[ctolkien]

Gah, PAT has expired. I'll fix this up shortly.

[ctolkien]

@manzanotti this is now available for testing on the GitHub Nuget repo here:
https://github.com/ctolkien/TinyPNG/pkgs/nuget/TinyPNG

I'll get another prerelease for Nuget.org out soon.

[manzanotti]

@manzanotti this is now available for testing on the GitHub Nuget repo here: https://github.com/ctolkien/TinyPNG/pkgs/nuget/TinyPNG

I'll get another prerelease for Nuget.org out soon.

Great, thanks. I might be able to test it this afternoon, otherwise it will be tomorrow, just working on something else at the moment.

[ctolkien]

@manzanotti

Great, thanks. I might be able to test it this afternoon, otherwise it will be tomorrow, just working on something else at the moment.

No worries. Build has gone for NuGet prerelease:
https://www.nuget.org/packages/TinyPNG/4.0.0-prerelease.68

Should show up soon ™️ once the NuGet indexing is done.

[manzanotti]

@ctolkien Pre-release works fine, thanks.