render empty section headers

Summary:Changed behavior so that empty section headers are rendered, fixes this [issue](https://github.com/facebook/react-native/issues/5639).
Closes https://github.com/facebook/react-native/pull/5713

Differential Revision: D3053765

Pulled By: sahrens

fb-gh-sync-id: 59a78a4b19288b2acc04a7b166de8c2ad16eacd7
shipit-source-id: 59a78a4b19288b2acc04a7b166de8c2ad16eacd7

Libraries/CustomComponents/ListView/ListViewDataSource.js

@@ -186,6 +186,11 @@ class ListViewDataSource {
       typeof this._sectionHeaderHasChanged === 'function',
       'Must provide a sectionHeaderHasChanged function with section data.'
     );
+    invariant(
+      !sectionIdentities || !rowIdentities || sectionIdentities.length === rowIdentities.length,
+      'row and section ids lengths must be the same'
+    );
+
     var newSource = new ListViewDataSource({
       getRowData: this._getRowData,
       getSectionHeaderData: this._getSectionHeaderData,
@@ -221,6 +226,10 @@ class ListViewDataSource {
     return this._cachedRowCount;
   }
 
+  getRowAndSectionCount(): number {
+   return (this._cachedRowCount + this.sectionIdentities.length);
+  }
+
   /**
    * Returns if the row is dirtied and needs to be rerendered
    */