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

Author: elicwhite

Libraries/Components/ScrollView/ScrollView.js

@@ -739,10 +739,7 @@ class ScrollView extends React.Component<Props, State> {
       this.props.contentOffset ? this.props.contentOffset.y : 0,
     );
     this._scrollAnimatedValue.setOffset(
-      /* $FlowFixMe(>=0.98.0 site=react_native_fb) This comment suppresses an
-       * error found when Flow v0.98 was deployed. To see the error delete this
-       * comment and run Flow. */
-      this.props.contentInset ? this.props.contentInset.top : 0,
+      this.props.contentInset ? this.props.contentInset.top || 0 : 0,
     );
     this._stickyHeaderRefs = new Map();
     this._headerLayoutYs = new Map();