[dbmanager] Fix settings default and Python type for connection with …

…postgis plugin

- Saved connections with empty password raised a QPyNullVariant parameter error due to lack of str default

python/plugins/db_manager/db_plugins/postgis/plugin.py

@@ -77,7 +77,7 @@ def connect(self, parent=None):
 		uri = QgsDataSourceURI()
 
 		settingsList = ["service", "host", "port", "database", "username", "password"]
-		service, host, port, database, username, password = map(lambda x: settings.value(x), settingsList)
+		service, host, port, database, username, password = map(lambda x: settings.value(x, "", type=str), settingsList)
 
 		# qgis1.5 use 'savePassword' instead of 'save' setting
 		savedPassword = settings.value("save", False, type=bool) or settings.value("savePassword", False, type=bool)