[BUGFIX] Logging PHP errors to sys_log table does not work

Logging PHP errors stopped working after changeset
3e309294 when the  DatabaseConnection
class was changed.
The changed line tries to determine, if a database connection is
available and only writes the log entry if the connection was
already established.

Resolves: #49645
Releases: 6.1, 6.2
Change-Id: Idee413ed911a8c2d23f034ceadd17c955c98a337
Reviewed-on: https://review.typo3.org/22166
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert

typo3/sysext/core/Classes/Error/ErrorHandler.php

@@ -167,7 +167,7 @@ class ErrorHandler implements \TYPO3\CMS\Core\Error\ErrorHandlerInterface {
 	 * @return void
 	 */
 	protected function writeLog($logMessage, $severity) {
-		if (is_object($GLOBALS['TYPO3_DB']) && !empty($GLOBALS['TYPO3_DB']->link)) {
+		if (is_object($GLOBALS['TYPO3_DB']) && $GLOBALS['TYPO3_DB']->isConnected()) {
 			$userId = 0;
 			$workspace = 0;
 			if (is_object($GLOBALS['BE_USER'])) {