🐛 Fix error in getItemCount with null items (#37)

* fixed error when items is null

in ImmutableVirtualizedList.js , in `getItemCount` when `items` is null throw error. With this change, at first it will checks `items` is valid and so on.

* added install scripts

* replace install by prepare in scripts

* Remove unrelated 'prepare' script

* Simplify logic

Author: mammad2c

src/ImmutableVirtualizedList/ImmutableVirtualizedList.js

@@ -114,7 +114,7 @@ class ImmutableVirtualizedList extends PureComponent {
         ref={(component) => { this.virtualizedListRef = component; }}
         data={immutableData}
         getItem={(items, index) => utils.getValueFromKey(index, items)}
-        getItemCount={(items) => (items.size || 0)}
+        getItemCount={(items) => ((items && items.size) || 0)}
         keyExtractor={(item, index) => String(index)}
         {...passThroughProps}
       />