Convert NULLs to NaN for Double/Floats

[Shimuuar]

SQLite stores NaN as NULL so we convert them back as NaN. Small program to illustrate issue.

go = withConnection ":memory:" $ \h -> do
  execute_ h "CREATE TABLE foo (x REAL)"
  execute h "INSERT INTO foo VALUES (?)" (Only (1::Double))
  execute h "INSERT INTO foo VALUES (?)" (Only ((0/0)::Double))
  mapM_ print =<< (query_ h "SELECT * FROM foo" :: IO [Only (Maybe Double)])
  mapM_ print =<< (query_ h "SELECT * FROM foo" :: IO [Only Double])

Output

*Main> go
Only {fromOnly = Just 1.0}
Only {fromOnly = Nothing}
*** Exception: ConversionFailed {errSQLType = "NULL", errHaskellType = "Double", errMessage = "expecting an SQLFloat column type"}

SQLite using same representation for NaN and NULL could cause some troubles. It makes impossible to distinguish between Just NaN and Nothing. So I'm not sure it is right thing to do.