Fix crash on null idp for SSO buttons (#8650) (#8653)

* 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>
(cherry picked from commit 60cd740b66)

Co-authored-by: Hugh Nimmo-Smith <hughns@users.noreply.github.com>

test/components/structures/auth/Login-test.tsx

@@ -146,4 +146,19 @@ describe('Login', function() {
         const ssoButtons = ReactTestUtils.scryRenderedDOMComponentsWithClass(root, "mx_SSOButton");
         expect(ssoButtons.length).toBe(3);
     });
+
+    it("should show single SSO button if identity_providers is null", async () => {
+        mockClient.loginFlows.mockResolvedValue({
+            flows: [{
+                "type": "m.login.sso",
+            }],
+        });
+
+        const root = render();
+
+        await flushPromises();
+
+        const ssoButtons = ReactTestUtils.scryRenderedDOMComponentsWithClass(root, "mx_SSOButton");
+        expect(ssoButtons.length).toBe(1);
+    });
 });