(Bug 50054) Fixed bug in migrateTrailingNLs DOM post-processor

* In an attempt to migrate a sequence of nodes starting with newlines,
  the code didn't check if a newline was found or not.

* Fixes parsing and RTing of all examples in the bug report (copied
  here as well):

[http://www.google.com Google <!-- comment text -->]
[[Main Page|Page<!-- comment text -->]]
<ref>[http://www.google.com Google <!-- comment text -->]</ref>

* No change in parser tests (TODO: Add more)

Change-Id: Ic4c5c8c8f87703e6e4fe18620075a891f8015fca