Fix an issue (Sticky header index was outside the range) when renderSeparator is defined but returns null

Summary:Fix an issue when using ListView and define `renderSeparator` but the implementation returns null.

In such cases the sectionHeaderIndices mismatch the child element index and the app shows a warning like "Sticky header index 18 was outside the range {0, 13}".
Closes facebook#5800

Differential Revision: D2980005

Pulled By: vjeux

fb-gh-sync-id: cd2d51d83698ed189bb65ea40b7b073644136b49
shipit-source-id: cd2d51d83698ed189bb65ea40b7b073644136b49

Author: christoph-jerolimov

Libraries/CustomComponents/ListView/ListView.js

@@ -394,8 +394,10 @@ var ListView = React.createClass({
             rowID,
             adjacentRowHighlighted
           );
-          bodyComponents.push(separator);
-          totalIndex++;
+          if (separator) {
+            bodyComponents.push(separator);
+            totalIndex++;
+          }
         }
         if (++rowCount === this.state.curRenderedRowsCount) {
           break;