[BUGFIX] Add missing (string) cast in dbal/DatabaseConnection

This is a regression fix for #54091.
It has been forgotten to cast the lastQuery to a string before
checking if it's empty

Resolves: #67040
Related: #54091
Releases: master
Change-Id: I6991cad799e686066f8205749baee4aed20a9127
Reviewed-on: http://review.typo3.org/39625


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>