About your issues.
The editor-UI is currently only able to preview the data from the first output of a node. For that reason does it not resolve them. A work-around for that is to add a No-Op node (or any other) to each of the outputs, as you already did. It will then resolve fine as that node has only one output and that output will have the data. All of that is just needed to display the data in editor-UI, if you execute the workflow normally it will also work fine without, so is a workable solution.
Yes, that is correct. If a node does not output any data it stops that branch. After all, if there is no data for the other nodes to execute they can not execute. That is also not an error. If you make a query to ClickUp that does not return any results, that is totally correct and no error. An error is if something goes wrong. if I have the numbers: 1,2,3,4 and then ask to give me the numbers that are larger than 5 and I get nothing in return, that is correct. Exactly the same here. Only if something goes wrong in the node execution and it can not do what it is supposed to do, like the ClickUp API can not be reached or returns an error, will the node error and then his option will have an effect. What you want is that it always outputs data. So you have to set the option “Always Output Data”. What n8n does then, is that if a node does not return any items, it returns an empty item anyway and the flow then keeps on executing.
Sorry, do not understand the part “then I have an empty object and then the IF node seems to be unable to determine which branch to take if I add one”. The IF-Node should then work totally fine. You just have to tell it what to do. You can set an expression to recognize an empty item and then route depending on that. Or you can set an expression which accesses the data from a previous node or a combination of both.
No idea why pressing “play” on the IF-Node would not work. But if you do that it, reuses existing data in the flow and it is quite possible that there is a bug somewhere and some things work not perfectly. So if you want to get the 100% correct result, you should press “Execute Workflow” that it executes from the start.