Make default proxy always available

If the user had an empty `file_transfer_proxies` config setting
no proxies were used. As we discover the default server proxy ourself the
user should not have to write it to his config setting to make use of it.

src/common/protocol/bytestream.py

@@ -519,9 +519,11 @@ class ConnectionSocks5Bytestream(ConnectionBytestream):
                 'file_transfer_proxies')
         shall_use_proxies = gajim.config.get_per('accounts', self.name,
                 'use_ft_proxies')
-        if shall_use_proxies and configured_proxies:
+        if shall_use_proxies:
             proxyhost_dicts = []
-            proxies = [item.strip() for item in configured_proxies.split(',')]
+            proxies = []
+            if configured_proxies:
+                proxies = [item.strip() for item in configured_proxies.split(',')]
             default_proxy = gajim.proxy65_manager.get_default_for_name(self.name)
             if default_proxy:
                 # add/move default proxy at top of the others