Empty scaladoc tags used to completely break the HTML layout of
classes and methods.

See the difference between before [1] and after [2].

[1]: http://static.gourlaysama.net/img/scaladoc_t5795_before.png
[2]: http://static.gourlaysama.net/img/scaladoc_t5795_after.png

Consider the following code:

/**
  * @see
  * @deprecated
  */
  object Foo

The comment parser properly parsed the body of the 'see' tag as
empty, but not the one of 'deprecated': it supposedly contains a single
character, a newline '\n', which is wrong.

This always happened to the last tag in the list; it is always appended
a new line (whether empty or not), which breaks formatting (and things
later on that test if a body is empty of not).

Empty scaladoc tags, like `@param`, `@return`, `@version`, etc. should
be omitted from the output when they have no meaning by themselves.

They are still parsed, for validation (warning that a tag doesn't
exist and so on), but are removed, if empty, when building the Comment.

The only ones that stay even when empty are `@deprecated`, so that the
class name can be striked-through ("Deprecated" also appears in the
header, even if there is no message with it), and `@throws`.