nir: Fix gnu-empty-initializer warning

Found with clang 14

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37615>

src/compiler/nir/nir_serialize.c

@@ -2349,7 +2349,7 @@ struct blob
 nir_validate_progress_setup(nir_shader *shader)
 {
    if (!NIR_DEBUG(PROGRESS_VALIDATION))
-      return (struct blob){};
+      return (struct blob){0};
 
    struct blob blob_before;
    blob_init(&blob_before);