docs: public static Predicate<String> notEmptyString() {
          return new Predicate<String>() {
              @Override
              public boolean test(String testValue) {
                  if (notNull().test(testValue)) {
                      for (Character c : testValue.toCharArray()) {
                          if (!Character.isWhitespace(c)) {
                              return true;
                          }
                      }
                  }
                  return false;
              }
          };
      }