Clean-up unnecessary variable name and associated API dox
Ensure merged paragraph style names are well defined

Though getAttributeNS is now standardised as of HTML4 to always return
null, according to MDN documentation browsers can return a variety of
values for a non-present attribute (generally null or ""). Update the
logic in TextController to always use "" for the merged style name as
the inline comment suggests is the expected behaviour.
Remove obsolete paragraph merge logic

As destination & source now never change, there is no need to re-
position the source paragraph. The source paragraph will now always be
checked for possible removal.
Merge pull request #562 from peitschie/getattr-fix

Ensure merged paragraph style names are well defined