Fixed KateBuffer::canEncode() never returning false

Note that the QStringEncoder::encode() methods in Qt6 don't actually encode
anything, but instead return lazy objects which are convertible to QByteArray.
The actual encoding happens during this conversion, hence we need to explicitly
force the conversion. Otherwise encoding will not happen, encoder.hasError()
will never return true, and the canEncode() method will never return false.

For details, see:
  * https://bugreports.qt.io/browse/QTBUG-117705
  * https://bugreports.qt.io/browse/QTBUG-117902



Signed-off-by: Jaak Ristioja <jaak@ristioja.ee>