Racket - NULL pointer handling

SWIG now converts a C/C++ NULL pointer into a null value by calling
scheme_make_null(), so that scheme's null? is true for a NULL C/C++
pointer value.

Consistency with Guile and needed for a pending commit for handling
NULL and std::unique_ptr.

Examples/test-suite/schemerunme/null_pointer.scm

@@ -0,0 +1,8 @@
+(define null '())
+(unless (funk null)
+  (error "funk(null) does not return true"))
+
+(unless (null? (getnull))
+  (error "NULL pointer should be null"))
+
+(exit 0)