Allow empty string for tabs -> title-format

CHANGELOG.asciidoc

@@ -96,6 +96,7 @@ Changed
   it caused some issues and then never re-applied.
 - Sending a command to an existing instance (via "qutebrowser :reload") now
   doesn't mark it as urgent anymore.
+- `tabs -> title-format` now treats an empty string as valid.
 
 Deprecated
 ~~~~~~~~~~

qutebrowser/config/configdata.py

@@ -650,7 +650,8 @@ def data(readonly=False):
             ('title-format',
              SettingValue(typ.FormatString(
                  fields=['perc', 'perc_raw', 'title', 'title_sep', 'index',
-                         'id', 'scroll_pos', 'host']), '{index}: {title}'),
+                         'id', 'scroll_pos', 'host'], none_ok=True),
+                 '{index}: {title}'),
              "The format to use for the tab title. The following placeholders "
              "are defined:\n\n"
              "* `{perc}`: The percentage as a string like `[10%]`.\n"

qutebrowser/mainwindow/tabwidget.py

@@ -106,7 +106,8 @@ class TabWidget(QTabWidget):
         fields['index'] = idx + 1
 
         fmt = config.get('tabs', 'title-format')
-        self.tabBar().setTabText(idx, fmt.format(**fields))
+        title = '' if fmt is None else fmt.format(**fields)
+        self.tabBar().setTabText(idx, title)
 
     def get_tab_fields(self, idx):
         """Get the tab field data."""