There are a few options, but if you wanted to use the Standard Mappings to accomplish this, you could add in a new mapping that sets the HL7 field to a blank string when it is 'null'.
For example, if the HL7 field is OBX-5, you would use a standard mapping to set OBX-5 to an empty string, and check the "Apply Condition" checkbox at the bottom of the Standard Mapping screen. That will allow you to set a condition that checks to see if OBX-5 is 'null'. If the return of that condition is true, OBX-5 would be replaced with an empty string. Here's a screenshot:
There are other ways to do this as well via a Custom Mapping, in which case you would use an if..else to evaluate if the JSON field was null before setting the HL7 field. If you'd like to look at that and other possible options, send an email in to support@qvera.com and we can help you with your specific situation.