fix(nextjs/ssr): parse params.asPath (#189)

There is an edge case where `params.asPath === '/'` and
gets converted to `{'/': null}` as search state.
Checking that we have an `?` before fix this issue.

packages/react-instantsearch/examples/next-app/pages/index.js

@@ -28,9 +28,9 @@ export default class extends React.Component {
      to initialize the searchState. 
   */
   static async getInitialProps(params) {
-    const searchState = qs.parse(
-      params.asPath.substring(params.asPath.indexOf('?') + 1)
-    );
+    const searchState = params.asPath.includes('?')
+      ? qs.parse(params.asPath.substring(params.asPath.indexOf('?') + 1))
+      : {}
     const resultsState = await findResultsState(App, { searchState });
     return { resultsState, searchState };
   }