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In the end I went with:

sync.from(extension.getFileProperty());

followed by:

pushTask.getFileInput().fileProvider(syncTask.map(s -> {
           ConfigurableFileTree files = project.fileTree(s.getDestinationDir());
           if (files.isEmpty()) {
             return null;
           } else {
             return files.getSingleFile();
           }
         }));

Then push task then has the following in it’s constructor:

onlyIf(t -> getFileInput().map(f -> f.getAsFile().exists()).getOrElse(false));