Fix null custom status logic for DurableTask.AzureStorage

As a part of a recent change to our serialization logic (see PR #465), we changed how null is serialized. This unforunately broke our logic for setting `null` CustomStatuses in DurableTask.AzureStorage.

This is because the `Merge` table operation we are performing ignores null parameters. The correct fix for this is to change from a `Merge` table operation to a `Replace` operation, but this will require a bit more work, as we would have to restructure how we generate the payload for this operation to ensure we don't break any other functionality.

In the meantime, we have incurred some technical debt by just sending along `"null"` when we detect a null value to preserve the original behavior.