[BUGFIX] Create workspace placeholder with processed field content

The field content that is used for the workspace placeholder records
is not processed through the field configuration.

This produces annoying error messages to the user.

Resolves: #17551
Releases: 6.2, 6.1, 6.0
Change-Id: I79823cc2bd0d26a03b0850045c08999ebdbc1a9b
Reviewed-on: https://review.typo3.org/24583
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring