Fix crash on null idp for SSO buttons (#8650)

* Add test case for null identity_providers for SSO

* Fix typing for identity_providers

* Make null idp explicit and handle in analytics

* chore: whitespace fix

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

src/Login.ts

@@ -51,7 +51,7 @@ export interface IIdentityProvider {
 export interface ISSOFlow {
     type: "m.login.sso" | "m.login.cas";
     // eslint-disable-next-line camelcase
-    identity_providers: IIdentityProvider[];
+    identity_providers?: IIdentityProvider[];
 }
 
 export type LoginFlow = ISSOFlow | IPasswordFlow;