1.6.1dev: follow-up to [17878], fix unable to save //not-configured//…

… in syntax highlighting of user preferences (refs #13864)

git-svn-id: http://trac.edgewall.org/intertrac/log:/branches/1.6-stable@17881 af82e41b-90c4-0310-8c96-b1721e28e2e2

trac/mimeview/pygments.py

@@ -174,7 +174,9 @@ def style_defs(style):
         if default_style not in styles:
             default_style = 'trac'
         selection = req.session.get('pygments_style')
-        if selection not in styles:
+        if not selection:
+            selection = ''
+        elif selection not in styles:
             selection = default_style
         output = self._generate('html', self.EXAMPLE)
         add_script_data(req, default_style=default_style, selection=selection)