[BUGFIX] Typecast value to string before applying trim()

Force value to be a string instead of null in
ConstantConfigurationParser to avoid exceptions.

Resolves: #90900
Releases: master
Change-Id: I968e86f95577eaa3d9d57e367cfb707f9a3e4ff5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64089


Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>