Identity server URL is empty when configured through .well-known without a custom identity server URL

[fridtjof]

Description

We are running Synapse v1.0.0, serving a .well-known/matrix/client endpoint that only contains a homeserver URL.
When entering a user name from our server into Riot Web (hosted at riot.im/app, whatever version that runs on right now - assuming latest as of right now), the home server is correctly configured, however the identity server is cleared. After logging in, the about section shows that the identity server URL is indeed an empty string.

Steps to reproduce

• Run a homeserver with a .well-known/matrix/client like this: {"m.homeserver": {"base_url": "https://my-homeserver-domain.org/"}}
• log into it through Riot (tested on riot.im/app only, but should be the same for all platforms)

I would expect Riot to default to its own identity server setting (in this case, vector.im) when a homeserver does not supply its own identity server.

Log: sent

Version information

• Platform: web

For the web app:

• Browser: Firefox 67.0.1
• OS: macOS 10.14.4
• URL: riot.im/app