C++: Handle add definition on empty files

Now in the case of a matching empty file we still select a best
token so the insertion happens *at* the end of the file.

The patch also fixes line breaks after the definition to be inserted
for situations in which there's no line break yet. Example:

namespace Foo {}

namespace {
}

Another thing for correctness. Member _key from HighestValue is now
value initialized instead of default initialized.

Task-number: QTCREATORBUG-6696
Change-Id: I5c0303675429c3da5cb88825272c9611be022f6a
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>