How to pass a null pointer to a C function

[Craig Dillabaugh]

Since questions about calling C from D seem to be popular today, 
I thought I would throw this one out there.

I am trying to call a C function which takes as parameters 
several arrays of doubles.  It is valid to have some arrays 
passed a NULL pointers in the C code.
To call this from D I've come up with the following, but it seems 
like a bit of a hack.

double[] x1 = [ 2.4, 3.7, 9.7, 4.5 ];
double[] y1 = [ 2.4, 9.8, 9.1, 3.4 ];
double[] empty;

SHPObject*[] shape_ptrs;
shape_ptrs ~= SHPCreateSimpleObject( SHPT_POLYGON, 
to!int(x1.length),
					   x1.ptr, y1.ptr, empty.ptr );

It should be clear what is going on.  A POLYGON is defined as a 
set of
points (provided as arrays of doubles), with X,Y and optional Z 
coordinates.
In this case I want a 2D polygon, so I want the final Z array to 
be
empty.

I wanted to just write 0 as the final parameter, but that didn't 
work, so
I tried to!(double*)(0), and that didn't make the compiler happy 
either.

I guess using the empty.ptr bit makes sense, in that the final 
array is meant to be empty, so pass it a pointer to an empty 
array.  But it seems a bit hackish that I need to declare an 
additional empty array just to call this function.

Is there an accepted 'proper' way of passing NULL pointers to C 
functions from D?