fix: prevent redirects to /undefined after saml auth

When a user authenticates using SAML, but their accounts aren't linked,
they land on the login page with the message to link their accounts. If the
`OC_REDIRECT_ON_TPA_UNLINKED_ACCOUNT` value is not set, they are redirected to
`/undefined`. This commit checks that the `redirectURL` is a valid non-empty
string before performing the redirect.

The bug was introduced in 91f8d36, so this can be safely dropped once that
temporary commit is removed from our branches. For Redwood, the relevant
commit hash is 64f5dea.

Internal-ref: https://tasks.opencraft.com/browse/BB-9010
(cherry picked from commit 9ade4ea)

lms/static/js/student_account/views/LoginView.js

@@ -268,7 +268,9 @@
                         this.clearFormErrors();
                         this.renderThirdPartyAuthWarning();
                     }
-                    window.location.href = redirectURL;
+                    if (typeof redirectURL === "string" && redirectURL.length) {
+                        window.location.href = redirectURL;
+                    }
                 } else {
                     this.renderErrors(this.defaultFormErrorsTitle, this.errors);
                 }