[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