hgweb: don't raise an exception when displying empty repos
The nullid node claims it's in the default branch, but the branch dict
is empty. This fixes the main symptom from issue696, but we may want
to set branchtags()['default'] = nullid somewhere for empty repos.
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'