Crash when trying to login to homeserver that does not return identity_providers field of m.login.sso flow schema

[hughns]

Describe the bug

Cinny is unable to sign in to a Matrix homeserver that offers the m.login.sso login type but where the homeserver does not provide a list of identity providers.

Instead the user ends up stuck on a blank screen.

Reproduction

1. Go to https://app.cinny.in/
2. Enter homeserver synapse-oidc.lab.element.dev into the input field
3. You end up on a blank screen
4. Check the Javascript console and see the following error:

index-cb0350e9.js:7 TypeError: Cannot read properties of undefined (reading 'sort')
    at hN (index-cb0350e9.js:57022:31396)
    at wy (index-cb0350e9.js:5:52909)
    at Cq (index-cb0350e9.js:9:8732)
    at Aq (index-cb0350e9.js:9:948)
    at Kv (index-cb0350e9.js:9:876)
    at mn (index-cb0350e9.js:9:729)
    at Pk (index-cb0350e9.js:7:10826)
    at index-cb0350e9.js:5:38721
    at g.unstable_runWithPriority (index-cb0350e9.js:1:13085)
    at XE (index-cb0350e9.js:5:38499)

Expected behavior

In the case that the homeserver does not specify a list of identity providers it should show a single SSO login button labelled something similar to "Continue with SSO".

Platform and versions

1. OS: macOS
2. Browser: Chrome 115.0.5790.114
3. App: 2.2.6 (app.cinny.in)
4. Homeserver: `synapse-oidc.lab.element.dev`

Additional context

The response to GET https://synapse-oidc.lab.element.dev/_matrix/client/v3/login is:

{
    "flows": [
        {
            "type": "m.login.password"
        },
        {
            "type": "m.login.sso",
            "org.matrix.msc3824.delegated_oidc_compatibility": true
        },
        {
            "type": "m.login.token"
        }
    ]
}

Which is valid as per the m.login.sso flow schema section of https://spec.matrix.org/v1.7/client-server-api/#client-login-via-sso because the identity_providers field is optional:

Optional identity providers (IdPs) to present to the user. These would appear (typically) as distinct buttons for the user to interact with, and would map to the appropriate IdP-dependent redirect endpoint for that IdP.