[BUGFIX] Check correctly for existing click event

With #90238 the JavaScript function `goToalt_doc` is registered as a
Fluid variable. However, the check for an existing event in the template
broke as the variable now always has a value.

To fix the situation, a new boolean variable `hasClickEvent` is
introduced.

Resolves: #90344
Related: #90238
Releases: master, 9.5
Change-Id: I7d86d300be923db82a0f79c3dd479a708230d92d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63185


Reviewed-by: Claus Due <claus@phpmind.net>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Richard Haeser <richard@maxserv.com>