Do not throw an error when dynamically branching over empty target

[psychelzh]

Sometimes, the targets to be dynamically branching over can be expected to be NULL (i.e., empty). Currently, an error is thrown upon this, which halts the whole workflow. I am looking forward to a warning/message but not an error. For example, branching over empty target just returns an empty target (say, NULL) with an warning/message.

[wlandau]

This special exception would require careful work to implement, and I do not think it makes sense to branch over an empty object anyway. If the object is empty, there is no need for branching at all. You can remove it from pattern = map(...) and supply the whole object directly to the command.

[wlandau]

So I plan to keep the existing error condition in targets.

[psychelzh]

Thanks. Maybe it is okay, then. For this edge case, we had better handle it manually.