Fix blob null handling during record retrieval

Blobs were failing during record updates when they were null.  The null
flag was set but the record still needed to be zeroed out.

SQL that used to fail:

CREATE TABLE `mytable` (
  `id` bigint(20) NOT NULL,
  `buddy_list_object` text,
  `focused_chat` int(11) unsigned NOT NULL DEFAULT '0',
  `recent_chats` text,
  `online_policy` tinyint(4) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=ROCKSDB DEFAULT CHARSET=latin1 ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8 COMPRESSION=ZLIB_STREAM COMMENT='recompression A-07-12';

INSERT INTO mytable (id, `recent_chats`) VALUES (1, 'foo') ON DUPLICATE KEY UPDATE `recent_chats` = VALUES(`recent_chats`);

INSERT INTO mytable (id, `recent_chats`) VALUES (1, 'foo') ON DUPLICATE KEY UPDATE `recent_chats` = VALUES(`recent_chats`);