NetworkManager VPN does not allow IPv6-only connections

When connecting to a VPN using NetworkManager (specifically tried using OpenConnect but the bug looks to be in the NetworkManager core code) with IPv4 disabled, connecting fails. NetworkManager attempts to get an IPv4 configuration anyway and fails if it isn't present.

On one VPN, with IPv4 disabled and no IPv4 DHCP in use, connecting failed because no IPv4 config could be found. With the same VPN profile, IPv4 disabled, but IPv4 addresses being handed out via DHCP, NetworkManager still looks for a connection and configures IPv4 even though it's disabled.

On a VPN with no IPv4 available, IPv4 disabled in NM:

Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <info>  [1539381667.4587] vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-65304bb12667,"VPN",0]: VPN connection: (IP Config Get) reply received.
Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <info>  [1539381667.4601] vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-65304bb12667,"VPN",8:(vpn0)]: VPN connection: (IP4 Config Get) reply received
Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <warn>  [1539381667.4604] vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-65304bb12667,"VPN",8:(vpn0)]: invalid IP4 config received!
Oct 12 15:01:07 jgoguen-fedora NetworkManager[1417]: <warn>  [1539381667.4606] vpn-connection[0x560695b500f0,05093305-9aac-4765-8a24-65304bb12667,"VPN",8:(vpn0)]: VPN connection: did not receive valid IP config information

On a VPN with IPv4 available, IPv4 disabled in NM, the connection succeeds and an IPv4 address is assigned.

With a wireless network, disabling IPv4 works as expected.