fix: prevent redirects to /undefined after saml auth (#669)

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.

Internal-ref: https://tasks.opencraft.com/browse/BB-9010

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

@@ -267,7 +267,9 @@
                         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);
                 }