Refactor ShareSheetExportFile

Summary:
Wanted to make a small change and found it a bit difficult to
follow, so I tried to make it a bit more TypeScript-y.

Logic should be unchanged.

Reviewed By: priteshrnandgaonkar

Differential Revision: D17762346

fbshipit-source-id: d0d8b2e25b532b7b10079907d6da86bdd878a75c

src/App.tsx

@@ -101,7 +101,7 @@ export class App extends React.Component<Props> {
             file={
               this.props.share && this.props.share.type === 'file'
                 ? this.props.share.file
-                : undefined
+                : null
             }
             logger={this.props.logger}
           />