code: public static <R> ImmutableGrid<R> of(int rowCount, int columnCount) {
          return new EmptyGrid<R>(rowCount, columnCount);
      }
docs: Obtains an empty immutable grid of the specified row-column count.
  
  @param <R> the type of the value
  @param rowCount  the number of rows, zero or greater
  @param columnCount  the number of columns, zero or greater
  @return the empty immutable grid, not null