UndefinedError when an error occurs if [project] admin_trac_url is empty

[Jun Omae]

[project] admin_trac_url in TracIni says;

Base URL of a Trac instance where errors in this Trac should be reported. This can be an absolute or relative URL, or '.' to reference this Trac instance. An empty value will disable the reporting buttons. (since 0.11.3)

However, stack trace is shown instead of disabling the reporting buttons. The issue occurs on both 0.12-stable and 1.0-stable.

...
  File "home/jun66j5/venv/py24-0.12/lib/python2.4/site-packages/genshi/template/eval.py", line 178, in evaluate
  File "/home/jun66j5/src/tracdev/git/trac/templates/error.html", line 102, in <Expression u'project.admin_href.newticket()'>
    <form class="newticket" method="get" action="${project.admin_href.newticket()}#">
  File "home/jun66j5/venv/py24-0.12/lib/python2.4/site-packages/genshi/template/eval.py", line 277, in _die
UndefinedError: None has no member named "newticket"

Originally reported at ​th:#9193 and ​th:comment:4:ticket:9988.