This script will concatenate a regular persons's name into a string while skipping null & empty strings.

dataweave-concatPersonsName.dwl

%dw 1.0
%output application/json
---
{
  result: [
    payload.person.firstName,
    payload.person.middleName, 
    payload.person.lastName
  ] 
  filter $ != null
  reduce ($$ ++ " " ++ $)
}