cli-plugins/manager: ListPlugins: return early if no candidates

Skip the other logic, which includes listing all commands provided; if
there's no plugin-candidates, those steps won't be needed.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli-plugins/manager/manager.go

@@ -151,6 +151,9 @@ func ListPlugins(dockerCli config.Provider, rootcmd *cobra.Command) ([]Plugin, e
 	}
 
 	candidates := listPluginCandidates(pluginDirs)
+	if len(candidates) == 0 {
+		return nil, nil
+	}
 
 	var plugins []Plugin
 	var mu sync.Mutex