CA_388624: fix(C SDK): fix build failure with recent GCC

The C SDK build was failing with a recent GCC on Fedora39 like this:
```
src/xen_common.c: In function ‘xen_session_logout’:
src/xen_common.c:298:5: error: ‘xen_call_’ accessing 16 bytes in a region of size 0 [-Werror=stringop-overflow=]
  298 |     xen_call_(session, "session.logout", params, 0, NULL, NULL);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Use `NULL` instead of a VLA of size 0.

Signed-off-by: Edwin Török <[email protected]>
(cherry picked from commit bfe78bd)

ocaml/sdk-gen/c/autogen/src/xen_common.c

@@ -292,10 +292,7 @@ set_api_version(xen_session *session)
 void
 xen_session_logout(xen_session *session)
 {
-    abstract_value params[] =
-        {
-        };
-    xen_call_(session, "session.logout", params, 0, NULL, NULL);
+    xen_call_(session, "session.logout", NULL, 0, NULL, NULL);
 
     if (session->error_description != NULL)
     {
@@ -314,10 +311,7 @@ xen_session_logout(xen_session *session)
 void
 xen_session_local_logout(xen_session *session)
 {
-    abstract_value params[] =
-        {
-        };
-    xen_call_(session, "session.local_logout", params, 0, NULL, NULL);
+    xen_call_(session, "session.local_logout", NULL, 0, NULL, NULL);
 
     if (session->error_description != NULL)
     {
@@ -336,29 +330,22 @@ xen_session_local_logout(xen_session *session)
 bool
  xen_session_get_all_subject_identifiers(xen_session *session, struct xen_string_set **result)
 {
-    abstract_value params[] =
-        {
-        };
 
     abstract_type result_type = abstract_type_string_set;
 
     *result = NULL;
-    xen_call_(session, "session.get_all_subject_identifiers", params, 0, &result_type, result);
+    xen_call_(session, "session.get_all_subject_identifiers", NULL, 0, &result_type, result);
     return session->ok;
 }
 
 
 bool
 xen_session_get_all_subject_identifiers_async(xen_session *session, xen_task *result)
 {
-    abstract_value params[] =
-        {
-        };
-
     abstract_type result_type = abstract_type_string;
 
     *result = NULL;
-    xen_call_(session, "Async.session.get_all_subject_identifiers", params, 0, &result_type, result);
+    xen_call_(session, "Async.session.get_all_subject_identifiers", NULL, 0, &result_type, result);
     return session->ok;
 }
 

ocaml/sdk-gen/c/gen_c_binding.ml

@@ -374,7 +374,9 @@ and impl_message needed classname message =
         sprintf
           "    xen_call_(session, \"%s.%s\", %s, %d, NULL, NULL);\n\
           \    return session->ok;\n"
-          classname message.msg_name param_call param_count
+          classname message.msg_name
+          (if param_count = 0 then "NULL" else param_call)
+          param_count
   in
 
   let messageAsyncImpl = impl_message_async needed classname message in