SectionList renderItem should be optional

Summary:
`renderItem` on `SectionList` is within the `OptionalProps` group of props but it is not actually marked as optional. Which means that doing things such as in the example where each section has its own `renderItem` and no `renderItem` prop is passed into `SectionList` will fail flow.

Create a `SectionList` where each section has it's own `renderItem` and do not pass in a `renderItem` into `SectionList`. Run flow, it should error.

[GENERAL] [MINOR] [SectionList] -Makes `renderItem` prop on `SectionList` optional for flow.
Closes https://github.com/facebook/react-native/pull/17262

Differential Revision: D6645672

Pulled By: hramos

fbshipit-source-id: 1096e8c4998c14003cf42f29ea559505082047c1

Libraries/Lists/SectionList.js

@@ -71,7 +71,7 @@ type OptionalProps<SectionT: SectionBase<any>> = {
   /**
    * Default renderer for every item in every section. Can be over-ridden on a per-section basis.
    */
-  renderItem: (info: {
+  renderItem?: (info: {
     item: Item,
     index: number,
     section: SectionT,