fix: use default empty style

[jaulz]

Summary

If style prop is undefined, StyleSheet.flatten will also return undefined. The code will dump consequently when it tries to destructure flatStyle:

react-native-safe-area-context/src/SafeAreaView.tsx

Lines 44 to 52 in fc98fb3

	const {
	margin = 0,
	marginVertical = margin,
	marginHorizontal = margin,
	marginTop = marginVertical,
	marginRight = marginHorizontal,
	marginBottom = marginVertical,
	marginLeft = marginHorizontal,
	} = flatStyle;

This is what I observed when using react-native-web.

Test Plan

The following code should work with this PR:

  <SafeAreaView>
    <Text>test</Text>
  </SafeAreaView>

[janicduplessis]

Thanks!