Reconsider not running convert functions against null values

[simonw]

I just got caught out by the fact that None values are not processed by the .convert() mechanism https://github.com/simonw/sqlite-utils/blob/0b7b80bd40fe86e4d66a04c9f607d94991c45c0b/sqlite_utils/db.py#L2504-L2510

I had run this code while working on #420 and I wasn't sure why it didn't work:

$ sqlite-utils add-column content.db articles score float
$ sqlite-utils convert content.db articles score '
import random
random.seed(10)

def convert(value):
    global random
    return random.random()
'

The reason it didn't work is that the newly added score column was full of null values.

I fixed it by doing this instead:

$ sqlite-utils add-column content.db articles score float --not-null-default 1.0

But this indicates to me that the design of convert() here may be incorrect.

[simonw]

Fixing this would require a bump to 4.0 because it would break existing code.

The alternative would be to introduce a new ignore_nulls=True parameter which users can change to ignore_nulls=False. Or come up with better wording for that.