Move prefs migration call from app-utils module load to inner_main

At the same time skip migration if the required xml and xsl files aren't found

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23275 57a11ea4-9604-0410-9ed3-97b8803252fd
Prefs migrate: skip preferences that don't have a value set

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23276 57a11ea4-9604-0410-9ed3-97b8803252fd