Both possible structures are considered. Here, I use the  examples from my previous posting:

set obj=##class(DC.Rick.MemberData).%OpenId(1)
do obj.%JSONExport() --> {"members":[{"dob":"1990-07-18","firstName":"Bob","memberId":123956}]}

set obj=##class(DC.Rick.MemberData).%OpenId(2)
do obj.%JSONExport() --> {}

The second example outputs {} only and not {"members":null}, I don't know why. Maybe there is a parameter which control this behavior, please ask WRC. 

From the view of data value, you can consider {} and {"members":null} as equal.

write {"members":null}.%GetTypeOf("members") --> null
write {}.%GetTypeOf("members") ----------------> unassigned

Both representation mean, the members property has no value. But, yes, but you can philosophize about it ...

I assume (according to the error message you show) you are trying to import some JSON-formatted data into an IRIS class. In addition I recommend the reading of https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cls...

To achieve this, you must define two IRIS classes:

Class DC.Rick.MemberData Extends (%Persistent, %JSON.Adaptor)
{
Property members As list Of Member;
}

Class DC.Rick.Member Extends (%SerialObject, %JSON.Adaptor)
{
Property dob As %Date;
Property firstName As %String;
Property middleName As %String;
Property nameSuffix As %String;
Property genderCode As %String;
Property lastName As %String;
Property memberId As %Integer;
Property relationship As %String;
}

Furthermore, I assume you have data like this (I shortened your example to keep things simple):

set memb0={"dob":"1990-07-18", "firstName":"Bob", "memberId":123956}
set memb1={"dob":"1990-05-25", "firstName":"Bill", "memberId":12345}
set memb2={"dob":"1990-10-30", "firstName":"Tommy", "memberId":4567}
set data(1)={"members":[(memb0)]}.%ToJSON()         // one member
set data(2)={"members":null}.%ToJSON()              // no member at all
set data(3)={"members":[(memb1),(memb2)]}.%ToJSON() // two members

check the examples:

for i=1:1:3 write data(i),!

the output should be:

{"members":[{"dob":"1990-07-18","firstName":"Bob","memberId":123956}]}
{"members":null}
{"members":[{"dob":"1990-05-25","firstName":"Bill","memberId":12345},{"dob":"1990-10-30","firstName":"Tommy","memberId":4567}]}

now import those data

for i=1:1:3 {
   set oref=##class(DC.Rick.MembersData).%New()
   if oref.%JSONImport(data(i)), oref.%Save() { write "OK",! } else { write "ERR",! }
}

If everything goes well, you should get three "OK"s and your data global looks like this

zwrite ^DC.Rick.MemberDataD
^DC.Rick.MemberDataD=3
^DC.Rick.MemberDataD(1)=$lb("",$lb($lb($lb(54620,"Bob","","","","",123956,""))))
^DC.Rick.MemberDataD(2)=$lb("","")
^DC.Rick.MemberDataD(3)=$lb("",$lb($lb($lb(54566,"Bill","","","","",12345,"")),$lb($lb(54724,"Tommy","","","","",4567,""))))

check member sizes:

for i=1:1:3 set oref=##class(DC.Rick.MemberData).%OpenId(i) write oref.members.Size,!

and the output should be:

1
0
2

I hope this is a good starting point for you...

It depends on...

Who is sitting at the other end? A Cache/IRIS server or a third-party product?

If Cache/IRIS: Mirroring, shadowing are the catchwords, you have to look for. In case of third-party SQL-DB: how fast (how often) want to do your updates? Once a day or (nearly)realtime?

I did something like that several years ago... the procedure is (just as a starting point):

Our application uses objects, so all the involved classes have an %OnAfterSave() method, something like this

Method %OnAfterSave(insert As %Boolean) As %Status
{
   do ..addToTransfer(..%Id())
}

with some smartness, like do not add if the record is already in the transfer queue, etc.  If you use SQL instead of objects, triggers are your friend.

We have also a task,  which crates (based on the class definition) a series of INSERT/UPDATE statement(s) and does the transfer with the help of  %SQLGatewayConnection.