BLOB/TEXT column 'abcdef' can't have a default value

MySQL in strict mode will error out with warning 1101:
BLOB/TEXT column 'abcdef' can't have a default value

We don't even seem to need this default value. Judging
from the code & from enwiki DB, it looks like no NULL
or '' ever even makes it in there & we can safely omit
the default value.

Change-Id: I2208d8d6252605908534443bc9f5c07aaea2a00b
Merge "BLOB/TEXT column 'abcdef' can't have a default value"