Be consistent with the number of lines

0 plugins -> empty string (instead of 2 lines)
1 plugin -> 1 line (instead of 2 lines)
2 plugins -> 2 lines (instead of 3 lines)

cmd/krew/cmd/list.go

@@ -51,7 +51,7 @@ Remarks:
 					names = append(names, name)
 				}
 				sort.Strings(names)
-				fmt.Fprintln(os.Stdout, strings.Join(names, "\n"))
+				fmt.Fprint(os.Stdout, strings.Join(names, "\n"))
 				return nil
 			}