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/guile/null_pointer_runme.scm

@@ -0,0 +1,3 @@
+(dynamic-call "scm_init_null_pointer_module" (dynamic-link "./libnull_pointer"))
+(load "testsuite.scm")
+(load "../schemerunme/null_pointer.scm")

Examples/test-suite/mzscheme/null_pointer_runme.scm

@@ -0,0 +1,3 @@
+(load-extension "null_pointer.so")
+
+(load "../schemerunme/null_pointer.scm")

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)