[BUGFIX] Fix initialization of TOTP info modal

The TOTP info modal displays the OTP auth url
along with a description. This content however
is added to the modal body by a callback. To
prevent the "modal content empty" warning
from being shown, the modal is now initialized
with an empty string as content.

Resolves: #94999
Releases: master
Change-Id: I0a8c274466d4f62da8edb26a4c2b547b66be3ba6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70761


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>