[BUGFIX] Handle absolute web paths in FormEngineUtility::getIconHtml

Since the introduction of always absolute (web) paths inside the TYPO3
backend, the TCA ctrl option `selicon_field` didn't work anymore since
relative web paths like `fileadmin/file.png` changed to
`/fileadmin/file.png`, which were not handled and resulted in a missing
icon.

This patch extends the condition for checking for absolute paths and
prevents the GU:getFileAbsFileName method from turning web paths into
empty strings.

Resolves: #95572
Resolves: #97442
Related: #95027
Releases: main, 11.5
Change-Id: I1b82f71fe4f0636bd786d102f361ca60c1294af9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77431


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>