@Nullable
    public ByteBuf buf() {
        return buf;
    }
Returns the {@link ByteBuf}.

@return the {@link ByteBuf}, or {@code null} if not created with
        {@link #DeframedMessage(ByteBuf, int)}.
grpc-protocol/src/main/java/com/linecorp/armeria/common/grpc/protocol/DeframedMessage.java