fix(configtree): returns error when no files are provided in import

The files argument in the import command is variadic. If you forget
specifying a tree name and pass only file to import, it will assume the
file as the tree name and the files tuple will be empty. It has to be
handled explicitly per the documentation here:
https://click.palletsprojects.com/en/8.1.x/arguments/

riocli/configtree/import_keys.py

@@ -100,6 +100,11 @@ def import_keys(
 
     Note: If --etcd-endpoint is provided, the keys are imported to the local etcd cluster instead of the rapyuta.io cloud.
     """
+    if not files:
+        spinner.text = click.style('No files provided.', fg=Colors.RED)
+        spinner.red.fail(Symbols.ERROR)
+        raise SystemExit(1)
+
     data, metadata = _process_files_with_overrides(files, overrides, spinner)
 
     if export_directory is not None: