code: @Nonnull
      public static <T> Collection<T> checkNotEmpty(Collection<T> collection, @Nonnull String errorMessage) {
          checkState(collection != null && !collection.isEmpty(), errorMessage);
          return collection;
      }
docs: Ensures a collection is neither null nor empty.
  
  @param collection collection to be validated.
  @param errorMessage error message to be thrown if collection is null or empty.
  @param <T> type of the collection item.
  @return collection passed in.