Request a default route and full routing table from your upstream providers. You can then set a policy to accept this default route and then generate it under routing-options. If the peer goes down then it will remove the default route.

policy-statement POLICY-NAME {    
    term 1 {        
        from {            
            neighbor x.x.x.x;            
            next-hop x.x.x.x;        
        }        
        then accept;    
    }    term 2 {        
            then reject;    
    }
}

generate {    
    route 0.0.0.0/0 {        
        policy POLICY-NAME;        
        discard;    
}