use templte on resource only if a resource is set. fix a TB

src/common/connection.py

@@ -78,6 +78,7 @@ class Connection(ConnectionHandlers):
 		self.password = passwords.get_password(name)
 		self.server_resource = gajim.config.get_per('accounts', name, 'resource')
 		# All valid resource substitution strings should be added to this hash. 
+		if self.server_resource:
 			self.server_resource = Template(self.server_resource).safe_substitute({
 				'hostname': socket.gethostname()
 			})