[BUGFIX] getPlainValue for @lazy objects

Typo3DbBackend::getPlainValue now loads lazy objects if they are
instances of LazyLoadingProxy

Fixes: #28149
Releases: 6.1, 6.0
Change-Id: Ibb7c342d2e0910190dcf414cef60a0e7d5f9d051
Reviewed-on: https://review.typo3.org/19913
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn