code: @Nullable
      public T computeIfAbsent(final Supplier<T> valueSupplier) {
          if (value == null) {
              value = valueSupplier.get();
          }
          return value;
      }
docs: If the value is not {@code null}, return it.
  If the value is {@code null}, attempts to compute its value using the given supplier function
  and set it into this Holder.
  
  @param valueSupplier the function to compute a value
  
  @return the current (existing or computed) value, or {@code null} if the computed value is {@code null}