NGINX does not re-start when configured with SSL certificate that depends on external SSL server that went down

I have an NGINX configuration with both HTTP and HTTPS traffic server blocks. Below is the HTTPS server block configuration snippet that is causing the problem.

server {
listen 10.1.1.5:443 default ssl;
listen 10.1.1.6:8080;

server_name myservice.traffic.dns.tmp;

ssl_certificate /etc/config/ssl/myservice.traffic.cert.pem;
ssl_certificate_key engine:name:myservice.traffic;
}

The external SSL server named "engine:name" is used to get the SSL certificate key. Initially, NGINX starts successfully. When the external SSL server goes down, if I try to restart NGINX, the restart fails because NGINX cannot connect with the external SSL server.

nginx: [emerg] ENGINE_load_private_key("385.1") failed
ENGINE_load_private_key:failed loading private key

This creates a "Hard" dependency on an external service.
How could I continue to serve my HTTP traffic even when the SSL external server is down? I would like to make nginx restart to succeed even when the external SSL server is down.

Thanks