Make setPagePrintSettings() accept IsLandscape again

Regression from 80c35d97 - invert
the check for boost::optional validity.

Change-Id: If4e041e1fe349c1fcb2c74b2e5780bf57300486f
Reviewed-on: https://gerrit.libreoffice.org/57309
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
(cherry picked from commit 0853b05b)
Reviewed-on: https://gerrit.libreoffice.org/57315Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
(cherry picked from commit cc124e47)

sw/source/uibase/uno/unotxdoc.cxx

@@ -1139,7 +1139,7 @@ void SwXTextDocument::setPagePrintSettings(const Sequence< beans::PropertyValue
         else if(sName == "IsLandscape")
         {
             auto b = o3tl::tryAccess<bool>(rVal);
-            bException = bool(b);
+            bException = !b;
             if (b)
             {
                 aData.SetLandscape(*b);