This may have been added functionality since you've posted this question, but maybe this is helpful to future readers :)
There is a setting in the Properties tab of your export mapping where you can have it to send empty values.
This should work for all data types, so decimals, dates, strings, etc.
In the export mapping open the mapping screen and select the microflow to convert your decimal to a string.
In the microflow the result should be something like this:
Hope this helps you a bit further in finding a solution.