Remove builder name so http://trac.webkit.org/changeset/106035 actually works.

Remove builder name so trac.webkit.org/changeset/106035 actually works.
https://bugs.webkit.org/show_bug.cgi?id=77164

Reviewed by Dirk Pranke.

http://trac.webkit.org/changeset/106035 depends on builder_name being
falsey by default. There's other code that requires it to be iterable
by default. So, make it be the empty string.

* Scripts/webkitpy/layout_tests/run_webkit_tests.py:
(parse_args):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106066 268f45cc-cd09-0410-ab3c-d52691b4dbfc