[TASK] Update phpdoc @return of exec_SELECTgetSingleRow()

DatabaseConnection::exec_SELECTgetSingleRow() may return FALSE if no row
is found. This is not reflected in the phpdoc for the return value.

This patch adds the necessary information to the doc header.

Resolves: #60172
Releases: 6.3, 6.2
Change-Id: Ia07868c2a1e8fad5b1d53d89ff0a959c2bcf9e3a
Reviewed-on: https://review.typo3.org/31490
Reviewed-by: Markus Klein
Tested-by: Markus Klein