Make mid() and midRef() properly return empty, non-null objects

Make mid() and midRef() properly return empty, non-null objects

If we request a substring starting at the very end of the string,
QString::mid should return an empty string, not a null string.
For instance, QString("abc").mid(3, 0) used to return a null
one, while this patch makes it return an empty one. The
same thing applies to QString::midRef() and QByteArray::mid().

Change-Id: Ie9efd7a0622d429efd0fb682c19856c19e9469af
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>