Fix new document formatting

[davidwengier]

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2317372

Presumably something changed in Roslyn where encoding can be null now? 🤷‍♂️

[davidwengier]

Oh cool, the test failure is real, so presumably this change broke things: https://github.com/dotnet/roslyn/pull/74623/files#r1865988171

@jasonmalinowski @CyrusNajmabadi do you want a follow up issue to re-look at the DocumentState change?

[CyrusNajmabadi]

@jasonmalinowski can yuo ptal :)

[davidwengier]

Running in to this in Razor, and it's been annoying me, so I'm going to merge this fix. If there is follow up, I'm very unlikely to be the right person to do the work anyway, so it wouldn't be in this PR regardless.