Fix bug where ScrollView contentInset top set to undefined wouldn't default to 0
Fix bug where ScrollView contentInset top set to undefined wouldn't default to 0

Summary:
If you passed
```
contentInset: { bottom: 10 }
```

then it wouldn't go into the if case and it could try to call `setOffset` with `undefined`. `setOffset` requires a number.

Changelog:
[Fix][ScrollView] ScrollView contentInset top now defaults to 0 in expected situations

Reviewed By: JoshuaGross

Differential Revision: D17796155

fbshipit-source-id: 951dbbb0de1052f64a6835963e8bbc564990c120

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