Вообще Variant - это довольно сложная структура. Вот её описание из паскалевского нелпа


TVarData represents the internal structure of a Variant variable.

Unit

System

const
varEmpty = $0000;
varNull = $0001;
varSmallint = $0002;
varInteger = $0003;
varSingle = $0004;
varDouble = $0005;
varCurrency = $0006;
varDate = $0007;
varOleStr = $0008;
varDispatch = $0009;
varError = $000A;
varBoolean = $000B;
varVariant = $000C;
varUnknown = $000D;
varString = $0100;
varAny = $0101;
varTypeMask = $0FFF;
varArray = $2000;
varByRef = $4000;

type
TVarArrayBound = packed record
ElementCount: Integer;
LowBound: Integer;
end;

PVarArray = ^TVarArray;
TVarArray = packed record
DimCount: Word;
Flags: Word;
ElementSize: Integer;
LockCount: Integer;
Data: Pointer;
Bounds: array[0..255] of TVarArrayBound;
end;

TVarData = packed record
VType: Word;
Reserved1, Reserved2, Reserved3: Word;
case Integer of
varSmallint: (VSmallint: Smallint);
varInteger: (VInteger: Integer);
varSingle: (VSingle: Single);
varDouble: (VDouble: Double);
varCurrency: (VCurrency: Currency);
varDate: (VDate: Double);
varOleStr: (VOleStr: PWideChar);
varDispatch: (VDispatch: Pointer);
varError: (VError: Longword);
varBoolean: (VBoolean: WordBool);
varUnknown: (VUnknown: Pointer);
varByte: (VByte: Byte);
varString: (VString: Pointer);
varAny: (VAny: Pointer);
varArray: (VArray: PVarArray);
varByRef: (VPointer: Pointer);
end;

Очевидно для значения null нужно чтобы поле TVarData.VType было равно varNull, т.е. $0001

Оно Вам надо - все это знать?