RUNTIME error: DEFAULT not allowed in this context

I'm using PostgresSQL and I wanted to update a column to the default value if it was NULL.
Beam happily accepts both
`(\r -> coalesce_ [just_ $current_ r.column] default_)`
and
`(\r -> if_ [isNothing_ (current_ r.column) `then_` default_] (else_ $ current_ r.column))`

But `coalesce(r.column,DEFAULT)` is not valid sql.
I need to supply the default manually.
I didn't find the proper way for this in beam, as the default is `current_timestamp` but the column is `timstamptz` in postgres corresponding to `UTCTime` in Haskell, but `now_` in Beam only works for `LocalTime` .
I got around it with using `coerce_ now_` instead,
but this doesn't feel right as it seems that GHC will happily coerce ANY beam type to one another...

(I have also used this coerce trick to get out of Maybe/Nullable when selecting only not null rows, haven't found a good way for this either.)