Strange behavior in TwoWire in Slave Mode #2937

Describe the bug
it looks like not possible to directly use TwoWire in slave mode. in this case the rxbuffer is never fill.
There is a différence in having an empty user receive handler and not having a receive handler. In the first case rxbuffer is fill correctly in the second case it is not fill.

Having:
TwoWire myI2C(PB7, PB6);
setup:
myI2C.begin(0x18) ;
in this case myI2C.available() is never > 0.

to have myI2C.available() > 0 when a message is received you must have implemented:
myI2C.onReceive(onReceiveHandler);

onReceiveHandler may be an empty code {}

in this situation TwoWire works correctly.

To Reproduce
TwoWire myI2C(PB7, PB6);
setup:
myI2C.begin(0x18) ;
loop:

if (myI2C.available()>0)
{
...
}

to make the program works:

setup:

myI2C.begin(0x18) ;
myI2C.onReceive(onReceiveHandler);


void onReceiveHandler(int howMany) {
  }