code: public static String checkNotEmpty(String arg, @Nullable Object errorMessage) {
      boolean empty = Strings.isNullOrEmpty(arg) || CharMatcher.whitespace().matchesAllOf(arg);
      checkArgument(!empty, errorMessage);
      return arg;
    }
docs: Ensures the argument is not null, empty string, or just whitespace.
  
  @param arg The argument
  @param errorMessage The exception message to use if the check fails
  @return The passed in argument if it is not blank