How to represent a null in a string?

I am trying to use a serial port utility in my vs 2005 vb project.  The syntax of the statement is outlined on their site as follows:
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Syntax 2
Value = object.Read ( StartTrigger, EndTrigger, Timeout )

Part Type Description
object SerialNET.Port An object that evaluates to an SerialNET.Port object.
Value string Data received from serial port. Nothing/null if no data received.
StartTrigger string.
EndTrigger string.
Timeout int See OnRead for details.
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Elsewhere in their limited documentation it says that I can set the "StartTrigger" string to null if I don't want to utilize a StartTrigger.  So, I tried structuring my code as:

psWork1 = objPort.read( "", EndTrig, 10000)

Where I was hoping the "" would act as a null....but it didn't.  And all I can get from the wiseguy in tech support is a lecture about how "" is the not the same as null....but he won't tell me HOW to represent a null in this "StartTrigger" string.

So....how do I do this?  Could I set up a string variable named StrTrig and put a "value" of null in it?  What would the actual syntax be?  TIAStart Free Trial