[BUGFIX] Respect empty alt attribute in ImageViewhelper

The ImageViewhelper does by default add the
`alternative` property as `alt` attribute to
the img tag if no non-empty string is set.

However, there might be valid use cases where
an empty `alt` attribute should be set for such
image - even if the property exists. Such case
is e.g. a thumbnail of such image.

The ImageViewhelper does therefore now render
an empty `alt` attribute if the `alt` argument
is explicitly set to an empty string.

Resolves: #102739
Releases: main, 12.4
Change-Id: Id197c3d2f839fbbc37744a2ae356c8e3ae35f0c4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82251


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>