Define semantics for CompoundTasks with no children

Follow up to: https://github.com/Azure/azure-functions-durable-js/pull/405/files#r1068072395

Our CompoundTask can be constructed without any sub-tasks / children. We handle this case by immediately setting the Task to "Completed" See here:

https://github.com/Azure/azure-functions-durable-js/blob/66ba4a172687543624cbc89a6c3836c70721a1e7/src/task.ts#L226-L247

This is an edge case, and perhaps one that we should not have supported in the first place. Assuming we were to continue supporting it, we should determine how to set the Task's `isPlayed` property, which is used to set the `context.IsReplaying` flag. Today, the tasks' `isPlayed` property remains unchaged, meaning that it is `false` and so it runs the risk of changing the `context.IsReplaying` flag to `false` when it shouldn't.

Start with the linked discussion and src/task.ts around lines 226-247, where an empty CompoundTask is immediately completed. Determine the intended isPlayed and context.IsReplaying semantics for this edge case, then establish the agreed behavior and update the implementation accordingly.

Written by the indexing model from the issue text.