[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>