Update QTextDocument::characterCount() for Qt 6

QtTextDocument::characterCount() return currently 1 even if the document is empty.

The QTextDocument will contain a single paragraph separator (unicode 0x2029)
when it is empty because it contains an empty block. This is a formatting
character, which is invisible, but is still counted as a character in the
document.

This is not documented well and is very misleading. 

Users have requested that characterCount should return 0 if the document is empty.