VirtualizedSectionList: Fix flow error about this.props.onViewableItemsChanged being null
VirtualizedSectionList: Fix flow error about this.props.onViewableItemsChanged being null

Summary:
Flow complains that the function could be null or undefined even after checking that because this._convertviewable is called first and could potentially muck with the type.
Here is an example in try flow: https://flow.org/try/#0CYUwxgNghgTiAEA3W8AeAueBvAZpg-ABSEB2ArgLYBGIMAlPALwB8SA9gJbB0C+A3ACgcZEmAAuHNiXg5CdTOWq1s8OGLIxpAZh4CBHHPEKoAdIYCEjeOQgQGWAfDRnCsunUE8gA

Changelog:
[Internal]

Reviewed By: jbrown215

Differential Revision: D17863358

fbshipit-source-id: 7689819de4c15f22e427cf5af520566bcea17bbc

myntra/react-native,janicduplessis/react-native,exponent/react-native,facebook/react-native,janicduplessis/react-native,myntra/react-native,exponentjs/react-native,hammerandchisel/react-native,pandiaraj44/react-native,exponent/react-native,hammerandchisel/react-native,javache/react-native,exponentjs/react-native,pandiaraj44/react-native,facebook/react-native,myntra/react-native,exponent/react-native,javache/react-native,pandiaraj44/react-native,myntra/react-native,myntra/react-native,exponentjs/react-native,javache/react-native,pandiaraj44/react-native,hoangpham95/react-native,myntra/react-native,exponent/react-native,exponent/react-native,javache/react-native,javache/react-native,arthuralee/react-native,pandiaraj44/react-native,hoangpham95/react-native,exponent/react-native,arthuralee/react-native,myntra/react-native,pandiaraj44/react-native,facebook/react-native,hoangpham95/react-native,facebook/react-native,myntra/react-native,pandiaraj44/react-native,janicduplessis/react-native,facebook/react-native,hammerandchisel/react-native,exponentjs/react-native,arthuralee/react-native,hoangpham95/react-native,janicduplessis/react-native,facebook/react-native,exponent/react-native,exponent/react-native,javache/react-native,facebook/react-native,arthuralee/react-native,javache/react-native,facebook/react-native,hoangpham95/react-native,janicduplessis/react-native,arthuralee/react-native,exponentjs/react-native,hoangpham95/react-native,exponentjs/react-native,janicduplessis/react-native,hammerandchisel/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,hammerandchisel/react-native,exponentjs/react-native,exponentjs/react-native,hoangpham95/react-native,myntra/react-native,hammerandchisel/react-native,janicduplessis/react-native,javache/react-native,hammerandchisel/react-native,hoangpham95/react-native,hammerandchisel/react-native,janicduplessis/react-native