Combine Fields

I would use 2 document level functions. The first would combine the fields but not include any field with a null value and include a separator value as needed. The other function would bet the field object and report any field that could not found by name.

https://www.dropbox.com/s/lqwo2clnb7pfjar/Concatinate%20Name%20and%20Address.pdf?dl=0

Concatenate Name and Address

This approach avoids extra spaces or other separators when there are fields with null strings for a value.

// Get the field values, as strings

var First_Name = getField("First_Name").valueAsString;

var Middle_Name = getField("Middle_Name").valueAsString;

var Last_Name = getField("Last_Name").valueAsString;

// Combine values, separated by a space

if(getField("Middle_Name").value == ""){

    event.value = First_Name + " " + Last_Name

}

else {

    event.value = First_Name + " " + Middle_Name + " " + Last_Name

}

This goes into the calculate> Custom Script and this script is if you don't have a middle name it wont look funny