[BUGFIX] MySQLi: FALSE not NULL from sql_fetch()/fetch_row()

MySQL previously returned FALSE if there were no more rows.
MySQLi returns NULL. So we'll need to turn that into FALSE
again not to break things.

Fixes: #46824
Releases: 6.1
Change-Id: Ia3f6e32ac71b9cec216459b66b42c7d9526671dd
Reviewed-on: https://review.typo3.org/19473
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn