How Do I Insert A Record With Possible NULL Values Using A Tableadapter.

I'm sorry, this question has a pretty long preamble, but bear with me as this will fully explain my problem, and save a number of clarification questions.
I am using Visual Web Developer 2005 Express, with SQL Server 2005. After reading a Microsoft article saying that it was much better to use a three tier architecture with data adapters than to write explicit queries, I have used this approach extensively, and generally it works very well.  Typical code patterns are: -
        Dim taPassenger As New ShippingTableAdapters.VPassengerTableAdapter
        Dim tbpassenger As Shipping.VPassengerDataTable
        Dim trpassenger As Shipping.VPassengerRow
        .....
        '   to process all passengers from a voyage: -
        tbpassenger = taPassenger.GetDataByVOYid(Voyageid)
        For Nbr As Integer = 1 To tbpassenger.Count
            trpassenger = tbpassenger(Nbr - 1)
             '    I now have the row available in properly-typed fields, eg trpassenger.vpxName
             ...
        Next
        '   to read and process a single passenger
        tbpassenger = tapassenger.GetDataByVPXid(VPXid)
        trpassenger = tbpassenger(0)
The table adapter has been created with Insert,Update, Delete methods.   Thus you can assign values to row fields, and then update the row: -
        trpassenger.VPXName = "New Name"
        ....
        tapassenger.update(trpassenger)
There are actually several overloads of the update method:  you could have written a field list, like this, where the names (VPXid etc) would be defined in my program as variables of the appropriate type for the corresponding column.
        tapassenger.update(VPXid, VPXName, VPXAge, ....
HOWEVER,  this doesn't work properly if you have non-string data that could be null.   Thus if the fourth field is a foreign key that could be null, if you write
        Dim VPXVoyId as Nullable(of GUID)
        VPXVoyid = nothing
        tapassenger.update(VPXid, VPXname, VPXAge, VPXVoyid, ...
or 
        Trpassenger.VPXVoyid = Nothing
        tapassenger.update(trpassenger)

you do NOT put a null value into the database.  Instead you store a value of "00000000-0000-0000-0000-000000000000".   You can solve this annoying problem in two ways: -
A.   If you will ALWAYS be storing a null value at this point of your program, then simply write the update statement like this: -
        tapassenger.update(VPXid, VPXname, VPXAge, Nothing, ...
Here "Nothing" actually means "DBNull" even though it doesn't mean this when used in a normal VB assignment (and neither does DBNull).
B   You can use the SetxxxxNULL method: -
        If trpassenger.VPXVoyid = nothing then
             trpassenger.SetVPXVoyidNull
        End If
        tapassenger.update(trpassenger) 


Thus when the program logic is complex and there are many paths to the update statement I will often put statements such as this in front of the update statement, using SetxxxxxNull for all non-string values that may be null.  You don't need to worry about string values because the Nullvalue property can be used to simply say that Null values are empty, and it all works properly, but for reasons unknown Microsoft have prohibited this for other data types.

Now at last we get to my question, which is "How do I do the equivalent for an Insert?"   tapassenger.insert does not have any overloads, and the only form is  tapassenger.insert(field list).   But this creates the problem above:  if VPXVoyid = nothing then I insert the not-null value of "00000000-0000-0000-0000-000000000000".   If I COULD use the form tapassenger.insert(trpassenger), then I'd have another problem: if you haven't set trpassenger (for example with trpassenger = tbpassenger(0)) then it will not exist, so you will get an exception when you try to reference it.  Yet you can't write
    If isnothing(trpassenger) then
       trpassenger = new shipping.vpassengerrow)         (I've tries this both as an assignment, and as a Dim statement)
    end if
So far I can think of only two solutions, both awful: -
a/    Use multi-choice logic to choose an appropriate insert statement.  If there is only one or two possibly-null fields, I could write:-
        If field4 = nothing then
            trpassenger.insert (VPXid, VPXname, VPXAge, Nothing, ..
        However if there is more than one possibly-null field, you need (2 to the power of  nbr-of-possibly-null-fields) insert statements, with of course a lot of testing to choose which one

b/    I could insert the record with nulls, then read it back and update it. 
Surely there is a better way!   Help!
 
Thank you, Robert Barnes.