Make onEndReachedThreshold=null match default value if prop not provided

Summary:
This change makes the behavior of providing a null value to onEndReachedThreshold match the behavior of not providing the prop at all.

## Changelog:

[General] [Changed] - Adjusted VirtualizedList's onEndReachedThreshold default value when null is provided

Reviewed By: olegbl

Differential Revision: D22870445

fbshipit-source-id: 29cbf0550702244007689759846ae0356f06b48d

Author: fatalsun

Libraries/Lists/VirtualizedList.js

@@ -1466,7 +1466,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     const distanceFromEnd = contentLength - visibleLength - offset;
     const threshold = onEndReachedThreshold
       ? onEndReachedThreshold * visibleLength
-      : 0;
+      : 2;
     if (
       onEndReached &&
       this.state.last === getItemCount(data) - 1 &&