Fix empty-line matching regexp in makeSeparator

* A fix for Bug 41756 now treats empty lines with comments
  consistently (doesn't matter whether the empty line has single
  or multiple comments).

* Parsoid's serializer hadn't caught up with this yet. This patch
  fixes the empty-line-matching regexp to conform with the mw-core
  change in that bug fix.

* No change in parser test results in this patch, but it eliminates
  some wt2wt and selser failures in my WIP patch for bug 50756.

Change-Id: I9c60e8ba7dbbbca47b3237564085eeeedce6647a
Merge "Fix empty-line matching regexp in makeSeparator"