[BUGFIX] Check for existence of t3js-login-url id in Login dialog

HTML element with identifier `t3js-login-url` is used to check whether
referrer handling is activated and suported. In case the `Login.html`
template has been overridden, mentioned element might not be given at
all - which leads to a corresponding JavaScript error.

Resolves: #91385
Releases: master, 9.5
Change-Id: Ie986a94209809c32cdfb217aa00b42f4369c525a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64484
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>

Build/Sources/TypeScript/backend/Resources/Public/TypeScript/Login.ts

@@ -114,7 +114,10 @@ class BackendLogin {
     const referrerRefresh = '#referrer-refresh';
     const referrerRefreshed = document.location.hash === referrerRefresh;
     const loginUrlWrapper = document.getElementById(this.options.loginUrlWrapper) as HTMLAnchorElement;
-    if (typeof loginUrlWrapper.dataset.referrerCheckEnabled === 'undefined' || parseInt(loginUrlWrapper.dataset.referrerCheckEnabled, 10) !== 1) {
+    if (loginUrlWrapper === null
+      || typeof loginUrlWrapper.dataset.referrerCheckEnabled === 'undefined'
+      || loginUrlWrapper.dataset.referrerCheckEnabled !== '1'
+    ) {
       return;
     }