Ldap\Connection: Return false if nothing is found for fetchRow()

This should behave like DbConnection::fetchRow().

refs #8954

library/Icinga/Authentication/User/LdapUserBackend.php

@@ -325,7 +325,7 @@ class LdapUserBackend extends LdapRepository implements UserBackendInterface
             throw new AuthenticationException('Connection not possible.', $e);
         }
 
-        if ($result === null) {
+        if ($result === false) {
             throw new AuthenticationException(
                 'No objects with objectClass "%s" in DN "%s" found. (Filter: %s)',
                 $this->userClass,

library/Icinga/Protocol/Ldap/Connection.php

@@ -235,7 +235,7 @@ class Connection implements Selectable
         $query->limit(1);
         $query->setUsePagedResults(false);
         $results = $this->fetchAll($query, $fields);
-        return array_shift($results);
+        return array_shift($results) ?: false;
     }
 
     /**