[fixed] allow null activeKey (empty) selection
[fixed] allow null activeKey (empty) selection

fixes #1227 I believe

Sipree/react-bootstrap,Terminux/react-bootstrap,react-bootstrap/react-bootstrap,jesenko/react-bootstrap,glenjamin/react-bootstrap,dozoisch/react-bootstrap,HPate-Riptide/react-bootstrap,glenjamin/react-bootstrap,pombredanne/react-bootstrap,react-bootstrap/react-bootstrap,apkiernan/react-bootstrap,mmarcant/react-bootstrap,Lucifier129/react-bootstrap,bbc/react-bootstrap,react-bootstrap/react-bootstrap,Lucifier129/react-bootstrap,egauci/react-bootstrap