XWIKI-11450: Title of Blog.BlogSheet does not get updated when updating from 6.1 to 6.x, x>1

* avoid triggering a merge in import just because
  the imported page has no syntax and thus falls back to xwiki/1.0
  while the default in the wiki is plain/1.0

(cherry picked from commit 90cb964e67546d9ee3c65146e1b597b84a55b5e1)