[BUGFIX] Avoid undefined array key warnings in PreviewUriBuilder

In class \TYPO3\CMS\Workspaces\Preview\PreviewUriBuilder the value
for TSconfig option "options.workspaces.previewPageId" is tested for a
syntax like field:value but if the actual value does not use a colon,
it will result in an undefined array key warning.

Resolves: #97169
Releases: main, 11.5
Change-Id: I08fc2865a7163e7770bf6ee56a24369b15ff0240
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73879


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>