Search'n'replace remaining NULL occurrences (#816)

modules/modpython/modpython.i

@@ -206,12 +206,12 @@ class CPyRetBool {
     PyObject* WriteBytes(PyObject* data) {
         if (!PyBytes_Check(data)) {
             PyErr_SetString(PyExc_TypeError, "socket.WriteBytes needs bytes as argument");
-            return NULL;
+            return nullptr;
         }
         char* buffer;
         Py_ssize_t length;
         if (-1 == PyBytes_AsStringAndSize(data, &buffer, &length)) {
-            return NULL;
+            return nullptr;
         }
         if ($self->Write(buffer, length)) {
             Py_RETURN_TRUE;