[TASK] Cast log message to string

PSR-3 expects loggers to accept objects as log message and to
cast them to string.

Resolves: #48877
Releases: 6.2
Change-Id: I7558b6db675704d67048914b79a7d8e405bc55d0
Reviewed-on: https://review.typo3.org/21804
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters