By default, Starfish will write a blank, "", value to the destination. If you instead want to write Null, you must use a script for this.
Function ScriptedField
ScriptedField = "@@ORG:law_firm_c@@"
if len(ScriptedField) = 0 then
ScriptedField = vbNullChar
end if
End Function
If you would prefer that no value at all gets written to the destination field, check the "Skip Blank Writes" box under Manage Stages for that stage.