fix UPDATE BYTES to default values

[nktks]

Hi!

I fixed case that updating BYTES default value when adding BYTES column with NOT NULL.

reproduce

schema diff

>   bytes BYTES(MAX) NOT NULL,

hammer diff

ALTER TABLE test ADD COLUMN bytes BYTES(MAX)
UPDATE test SET bytes = '' WHERE bytes IS NULL
ALTER TABLE test ALTER COLUMN bytes BYTES(MAX) NOT NULL

hammer apply error

Error: rpc error: code = InvalidArgument desc = Value of type STRING cannot be assigned to bytes, which has type BYTES [at 1:25]\nUPDATE test SET bytes = \'\' WHERE bytes IS NULL\n

[daichirata]

thank you so much 👍