iOS: Avoid adding extra spaces to accessibility label

[rigdern]

In some cases, RCTRecursiveAccessibilityLabel could return an accessibility label that had leading space, trailing space, or multiple spaces between words. This is because it always added a space before adding a label even if the label turned out to be empty.

This is fixed by being stricter about adding spaces.

Test Plan (required)

Found test cases that used to introduce leading space, trailing space, or multiple spaces between words and verified that there aren't any extra spaces after the fix.

{/* Used to have leading space */}
<View accessible={true}>
  <View />
  <View accessibilityLabel='Two' />
  <View accessibilityLabel='Three' />
</View>

{/* Used to have 2 spaces between "One" and "Three" */}
<View accessible={true}>
  <View accessibilityLabel='One' />
  <View />
  <View accessibilityLabel='Three' />
</View>

{/* Used to have trailing space */}
<View accessible={true}>
  <View accessibilityLabel='One' />
  <View accessibilityLabel='Two' />
  <View />
</View>

Additionally, my team is using this fix in our app.

Adam Comella
Microsoft Corp.

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[shergin]

@rigdern Thank you!