Add nullability annotations to DaprClient

Describe the proposal

Once we ingest the .NET 5.0 SDK we we’ll be able to add nullability annotations to most of the methods on DaprClient. The reason we didn’t do this because is because prior to the C#9 compiler, a core use case didn’t work with nullability:

public Task<TValue> DoSomething<TValue>();

There is no supported way (pre C# 9) to say this method returns a task that might return a null without also constraining TValue : class. We didn’t want to block the use of value types at a fundamental level especially when records and structs are about to hit.

Now that this barrier has been removed there will be a benefit to adding NNRT annotations.

[philliphoff]

@halspang Do you think there’s any interest in this still? I just ran into a bug in my own application as a result of a wayward null that would have been caught with proper annotations.

[dlemstra]

The where T : default solution seemed to work @rynowak. I also created the first PR for this with one of the small libraries. Can you clarify what you mean with the bindings?