How do I write Null attribute values to Salesforce?

I would try the NullAttributeMapper transformer to set the nulls and route the output to the Data Inspector first to ensure that the fields appear there as <null>

Then write the data to Salesforce. I know Salesforce supports null, and I see no indication that it's not supported by FME, so my expectation is that the right thing will just happen.

If that's not the case, please let us know via our support team - safe.com/support