Support null-children in Section.
Support null-children in Section.

Fixes #10.
Purii/react-native-tableview-simple,beaurushton/react-native-tableview-simple,Purii/react-native-tableview-simple,Purii/react-native-tableview-simple,beaurushton/react-native-tableview-simple,Purii/react-native-tableview-simple,beaurushton/react-native-tableview-simple