The customer wanted any documentation stating the expected behavior is not to install routes on the bgp.l3vpn.0 table when there are no vrf routing instances configured 

No routes are been installed on bgp.l3vpn.0 table when there are no vrf routing instances configured

There is no such documentation but that is because that statement is implied on how bgp works, for the routes to be shared to other PEs and installed in their respective bgp.l3vpn.0 tables, there must be a matching vrf policy, which is configured under the routing-instance, for the routes to be installed on this table:

"bgp.l3vpn.0—Stores routes learned from other PE routers. Routes in the bgp.l3vpn.0 routing table are copied into a Layer 3 VRF when there is a matching VRF import policy in the PE router. This table is present only on PE routers, and it does not store routes received from directly connected CE routers.

https://www.juniper.net/documentation/us/en/software/junos/vpn-l3/topics/topic-map/l3-vpns-routes-vrf-tables.html"

https://www.juniper.net/documentation/us/en/software/junos/vpn-l3/topics/topic-map/l3-vpns-routes-vrf-tables.html"

https://www.juniper.net/documentation/us/en/software/junos/vpn-l3/topics/topic-map/l3-vpns-overview.html