Task common files should support nullable enable

[ViktorHofer]

I'm explicitly disabling the nullable checks in the src/tasks/common files via #nullable disable statements so that they don't show up as error in nullable enabled projects that include them. These files should be updated to support nullability. This won't cause issues when non nullable enabled projects will import them as the nullable feature is purely additive.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[baronfel]

We should do this as part of an all-up wave of nullability support in the SDK - I'm not sure what our current state is. Will chat with the team about if this is a thing we want to do all-up.

[marcpopMSFT]

@v-wuzhai I see 63 instances of nullable disabled. We aren't sure how much work it would be to remove all of these. Can you try removing a few and fixing up the code for any errors that show up? @MiYanni will help you with the reviews. You don't have to fix all 63 at once and if this ends up looking like a massive effort, we can potentially delay this.