Why do you think you need this? As far as I can tell this is beyond the scope of what you'd do in Designer; it doesn't matter if there is any input, you simply process whatever data there is however you want to, then let the end user deal with the data being present or not.
It's a procedural environment, ergo you set up procedures to "do" stuff but without bothering yourself if something is actually coming in. You mention FX Map but it doesn't do this either: it's not FX Map but Designer itself which notices multiple inputs and then based on that applies differences to the node and some of its properties.
See, if you would apply a function to the image selector within FX Map then all it can do is ... select a specific image, but the function will have no knowledge what so ever as to how many inputs are available. Like I said: this is beyond the scope of Designer.