Upload empty content for binary files to set correct upload status.

Fix for Issue 54: http://code.google.com/p/rietveld/issues/detail?id=54

Holding back in favor of http://codereview.appspot.com/6455
and http://codereview.appspot.com/7263/show

Currently no base content for binary files (except images, of course) is uploaded to Rietveld. A Content instance is created but since there's no upload Content.text remains None which causes a 404 when viewing sbs.

This patch sets the default for base_content from None to "" (empty string) in upload.py.

To reproduce this issue:

[go to Rietveld's trunk directory]
$ cp django.zip foo.zip
$ svn add foo.zip
$ upload.py -s localhost:8080 -y -m "test issue54"

[JohnA, 2008-10-02 20:40:25]

hmm, instead of doing this (which slows upload and might confuse a user), why
not just have content.text default to an empty string instead of None?

[JohnA, 2008-10-02 21:36:39]

nm, my suggestion still leaves an empty status.  LGTM.