[BUGFIX] Don't rely on not found database records impexp

The function exec_SELECTgetSingleRow could also return
FALSE in addition to NULL, so add a additional check.

Resolves: #58688
Resolves: #57348
Releases: 6.2
Change-Id: Ic04071f67f02cce12b11c34d46c084bc28ccc83a
Reviewed-on: https://review.typo3.org/29993
Reviewed-by: Marc Bastian Heinrichs
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel