QTextCodec::toUnicode() behavior changed

Moving from Qt5 to Qt6, QTextCodec::toUnicode() behavior is changed. 

In Qt6 QTextCodec::codecForName("ISO 8859-1")->toUnicode("",0) returns
QString()
but it should return
QString(0, QChar(0))
Or in other words it should return of Data::allocate(0) instead of Data::sharedNull()

Attached example reproduces this. When run example output:

#Qt5.15: ###

input:  "" isNull:  no isEmpty:  yes
input:  "", 0 isNull:  no <–(behavior) isEmpty:  yes
input:  nullptr isNull:  yes isEmpty:  yes

#Qt6.5.2: ###

input:  "" isNull:  no isEmpty:  yes
input:  "", 0 isNull:  yes <–(different behavior)isEmpty:  yes
input:  nullptr isNull:  yes isEmpty:  yes

Expectation is, toUnicode("") and toUnicode("",0) should return the same value and an empty string should stay an empty string no matter which coding is used.

Update:

probably fix also needed in qtextcodec.h

QString toUnicode(const char *in, int length, ConverterState *state = nullptr) const
{
if (in == nullptr || length <= 0)
return {};
return convertToUnicode(in, length, state);
}
QByteArray fromUnicode(const QChar *in, int length, ConverterState *state = nullptr) const
{
if (in == nullptr || length <= 0)
return {};
return convertFromUnicode(in, length, state);
} 

"|| length <= 0" may be removed.