SQL/ODBC: Don't assert when no error record is available

When SQLGetDiagRec() does not return an record, the list of
DiagRecords might be empty. This will create an assertion when trying
to access QList::front() or similar. Therefore we need to check if the
list is empty before accessing it.

This amends 4ec5c0efc756a39162b43367438fee965c229ae7

Fixes: QTBUG-122073
Pick-to: 6.6
Change-Id: I6f421d82f9b6fdf84672d755cbbe8d2adec13266
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 6287daaa20bfb306d1e2d87d67df961e1da2857a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/sqldrivers/odbc/qsql_odbc.cpp

@@ -269,6 +269,8 @@ static DiagRecord combineRecords(const QList<DiagRecord> &records)
                           a.sqlState + u';' + b.sqlState,
                           a.errorCode + u';' + b.errorCode};
     };
+    if (records.isEmpty())
+        return {};
     return std::accumulate(std::next(records.begin()), records.end(), records.front(), add);
 }