suppress -Werror=empty-body in char_traits implementation (#3098)

Author: ericniebler

libcudacxx/include/cuda/std/detail/libcxx/include/__string

@@ -263,7 +263,8 @@ struct _CCCL_TYPE_VISIBILITY_DEFAULT char_traits<char>
 #if _CCCL_COMPILER(GCC, <, 13)
     // absurd workaround for GCC "internal compiler error: in cxx_eval_array_reference"
     if (_CUDA_VSTD::is_constant_evaluated())
-      ;
+    {
+    }
 #endif
 #if defined(_CCCL_BUILTIN_STRLEN)
     NV_IF_ELSE_TARGET(NV_IS_DEVICE,