qtwasmserver accepts a positional path argument which tells where to
serve the assets from. The argument wasn't actually used, and this
resulted in always using the cwd.
In addition add a check for the path directory existence. Otherwise
this becomes only visible as a 404 Not Found error.
Amends: 156e5c8b690d01ad3043d2163168c4ea3608a046
Pick-to: 6.9
Fixes: QTBUG-134393
Change-Id: Iacfafe8a2fb2409169b09a17dbc9ffed0ad16fdf
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
fcada7c5c3