Lists: Change flow type to not allow rendering undefined

Summary:
`?React.Element<any>` allows passing in `undefined`, an invalid value to render into a React component. Changing these types to be `null | React.Element<any>`.

The issues that this caught were fixed in a previous diff.

Reviewed By: lunaleaps

Differential Revision: D17859220

fbshipit-source-id: 71438cb357b44bca0bf3437aea99ece99a616f7d

Libraries/Lists/SectionList.js

@@ -53,7 +53,7 @@ type OptionalProps<SectionT: SectionBase<any>> = {
       unhighlight: () => void,
       updateProps: (select: 'leading' | 'trailing', newProps: Object) => void,
     },
-  }) => ?React.Element<any>,
+  }) => null | React.Element<any>,
   /**
    * Rendered in between each item, but not at the top or bottom. By default, `highlighted`,
    * `section`, and `[leading/trailing][Item/Separator]` props are provided. `renderItem` provides
@@ -146,11 +146,15 @@ type OptionalProps<SectionT: SectionBase<any>> = {
    * Rendered at the top of each section. These stick to the top of the `ScrollView` by default on
    * iOS. See `stickySectionHeadersEnabled`.
    */
-  renderSectionHeader?: ?(info: {section: SectionT}) => ?React.Element<any>,
+  renderSectionHeader?: ?(info: {
+    section: SectionT,
+  }) => null | React.Element<any>,
   /**
    * Rendered at the bottom of each section.
    */
-  renderSectionFooter?: ?(info: {section: SectionT}) => ?React.Element<any>,
+  renderSectionFooter?: ?(info: {
+    section: SectionT,
+  }) => null | React.Element<any>,
   /**
    * Makes section headers stick to the top of the screen until the next one pushes it off. Only
    * enabled by default on iOS because that is the platform standard there.

Libraries/Lists/VirtualizedSectionList.js

@@ -41,7 +41,7 @@ export type SectionBase<SectionItemT> = {
       unhighlight: () => void,
       updateProps: (select: 'leading' | 'trailing', newProps: Object) => void,
     },
-  }) => ?React.Element<any>,
+  }) => null | React.Element<any>,
   ItemSeparatorComponent?: ?React.ComponentType<any>,
   keyExtractor?: (item: SectionItemT, index?: ?number) => string,
 };
@@ -71,15 +71,15 @@ type OptionalProps<SectionT: SectionBase<any>> = {
       unhighlight: () => void,
       updateProps: (select: 'leading' | 'trailing', newProps: Object) => void,
     },
-  }) => ?React.Element<any>,
+  }) => null | React.Element<any>,
   /**
    * Rendered at the top of each section.
    */
-  renderSectionHeader?: ?({section: SectionT}) => ?React.Element<any>,
+  renderSectionHeader?: ?({section: SectionT}) => null | React.Element<any>,
   /**
    * Rendered at the bottom of each section.
    */
-  renderSectionFooter?: ?({section: SectionT}) => ?React.Element<any>,
+  renderSectionFooter?: ?({section: SectionT}) => null | React.Element<any>,
   /**
    * Rendered at the bottom of every Section, except the very last one, in place of the normal
    * ItemSeparatorComponent.