fix: check for nil example

[Freebien]

Hi I had a panic with a nil pointer dereference when trying to set a Header without an example.

Here is a fix that checks the existance of the example before getting the raw info. (maybe the issue is more on the gnostic side, but since it's some generated code, I don't know how to work with it).

Here is an example that does the panic.

syntax = "proto3";
package auth.login;

option go_package = "gen/auth";

import "gnostic/openapi/v3/annotations.proto";

service AuthService {
    rpc Login(LoginRequest) returns (LoginResponse) {
        option (gnostic.openapi.v3.operation) = {
            responses: {
                response_or_reference: [{
                    name: "200",
                    value: {
                        response: {
                            description: "Login success"
                            headers: {
                                additional_properties: [{
                                    name: "Set-Cookie"
                                    value: {
                                        header: {
                                            description: "header description"
                                            /* if this is commented, it panics
                                            example: {
                                                yaml: "header example"
                                            }
                                            */
                                            schema: {
                                                schema: {
                                                    type: "string"
                                                    example: {
                                                        yaml: "schema example"
                                                    }
                                                }
                                            }
                                        }
                                    }
                                }]
                            }
                        }
                    }
                }]
            },
            security: [
                {
                    additional_properties: []
                }
            ]
        };
    };
}

message LoginRequest {
    string username = 1;
    string password = 2;
}

message LoginResponse {
    bool success = 1;
}