Fix some minor bad use of strings in WebCore
https://bugs.webkit.org/show_bug.cgi?id=114907

Reviewed by Darin Adler.

* editing/Editor.cpp:
(WebCore::Editor::selectedText): This was implicitly getting the emptyString()
for the argument "".
* editing/markup.cpp:
(WebCore::StyledMarkupAccumulator::takeResults): ditto.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148854 268f45cc-cd09-0410-ab3c-d52691b4dbfc