Fix nullable annotations in *Operations

[campersau]

Some parameters can be null like

ContainersPruneParameters parameters = null

so the parameters should be marked as nullable

ContainersPruneParameters? parameters = null

Also replaced default(CancellationToken) with default for consistency.

Changes only affect *Operations interfaces / classes.

[HofmeisterAn]

Thanks for the PR. I have two quick questions.

Some operation interfaces still use ... CancellationToken cancellationToken);. Probably something we should align too?

[HofmeisterAn]

I haven't checked all of them, but it seems the interface sometimes uses different signatures (nullable) compared to the implementation. The interface looks like this (see imageStream, headers):

Task CreateImageAsync(ImagesCreateParameters parameters, Stream imageStream, AuthConfig authConfig, IDictionary<string, string> headers, IProgress<JSONMessage> progress, CancellationToken cancellationToken = default);

[campersau]

Yes, I have made some arguments nullable in the implementation (since they can be) to avoid nullable warnings.
The implementation can actually allow null even though it is not defined in the interface.

If you want I can mark all arguments nullable when they can actually be null in the implementation.

[HofmeisterAn]

If you want I can mark all arguments nullable when they can actually be null in the implementation.

That's probably the cleanest approach. WDYT?

[campersau]

Some operation interfaces still use ... CancellationToken cancellationToken);. Probably something we should align too?

I have fixed them. There were just 3 in the interfaces and two of them are actually obsolete.

[campersau]

Also some interfaces are implemented explicitly and others are not. If you want I can also make them consistent (either way).

[HofmeisterAn]

Also some interfaces are implemented explicitly and others are not. If you want I can also make them consistent (either way).

If we change it, it would be great to align and make it consistent (as long as it's not too much work). I don't think there's a need to implement them explicitly?