Web Service returned fields exceptions are not informing properly about null values

I've seen the following a couple of times reported in the forums:

Invalid response value detected: password => Invalid response value detected: Invalid external api response: the value is "", the server was expecting "raw" type"

In this case, the value is not really an empty string, the value is a NULL value and depending on how the exporter or external field is defined, it may not allow NULL values.

Please, note that this mostly happens in sites with the DB schema broken or incorrect upgrades where there are NULL values where they shouldn't be.