Right now if there is a session config but it has no data in it,
`KMainWindow::canBeRestored()` will return true if you call it with the
number 1 as an argument. This is because it will try to read the
NumberOfWindows key, see that it does not exist, and fall back to the
number 1 as the default value, which will be identical to the number you
called the function with.

Instead let's set the default value to 0, which will fix this and also
be more accurate too (there really are zero windows saved).