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.

Examples/test-suite/octave/null_pointer_runme.m

@@ -6,3 +6,13 @@ endif
 null_pointer;
 
 assert(funk([]));
+null_ptr = getnull();
+assert(ismatrix(null_ptr))
+assert(size(null_ptr) == size([]))
+assert(isequal([], null_ptr))
+
+# Can't use isnull as a test due to null matrix not being copyable...
+#   n = []
+#   isnull(n) # ans = 0
+#   isnull([]) # ans = 1
+#   isnull(getnull()) # ans = 0