Octave - SWIG now marshalls a C/C++ NULL pointer into the null matrix, []

SWIG has always marshalled the null matrix into a NULL pointer; this remains
and now we have consistency in representing a NULL pointer.

This is a pre-requisite for a pending commit to fully support std::unique_ptr.

Doc/Manual/Octave.html

@@ -363,6 +363,10 @@ octave:2> f=swigexample.fopen("not there", "r");
 error: value on right hand side of assignment is undefined
 error: evaluating assignment expression near line 2, column 2 </pre></div>
 
+<p>
+NULL C/C++ pointers are represented by the Octave null matrix, <tt>[]</tt>.
+</p>
+
 <H3><a name="Octave_nn13">30.3.6 Structures and C++ classes</a></H3>