Fix MacDive import to convert Imperial temperature

MacDive import needs to convert temperatures to Celcius. However, it
seems that MacDive reports no reading as 32F (and probably 0C when using
Metric). There is no way of knowing whether we have no reading or
the temperature is actually zero, so we use the given temperature
currently as 0C is a valid temperature...

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>