[bugfix] custom_browser is NoneType when not set

Got the following traceback when opening the PreferenceDialog for the first time 
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/turpial/ui/qt/main.py", line 278, in show_preferences_dialog
    self.preferences_dialog = PreferencesDialog(self)
  File "/usr/lib/python2.7/site-packages/turpial/ui/qt/preferences.py", line 44, in __init__
    self.browser_page = BrowserPage(base)
  File "/usr/lib/python2.7/site-packages/turpial/ui/qt/preferences.py", line 256, in __init__
    self.command.setText(current_browser)
TypeError: QLineEdit.setText(QString): argument 1 has unexpected type 'NoneType'

The commit fixes the faulty type