[iOS] Fabric: Fixes high contrast color when not specified

[zhongwuzw]

Summary:

Fixes the high contrast color when not specified. Previously, if we didn't specify highContrastLight or highContrastDark, it would create a UIColor with an alpha of 0, which is not what we expected. We should fall back to the dark or light color instead.

Changelog:

[IOS] [FIXED] - Fabric: Fixes high contrast color when not specified

Test Plan:

1. Enable color contrast in iPhone's settings.
2. Open PlatformColor example in RNTester. See dynamic colors
3. color shows correctly
   

Before:

[zhongwuzw]

@cipolleschi Hi, can you help me to review this? :)

[cipolleschi]

Grat catch... creating a UIColor from 0 probably returns a black color rather than null..

[facebook-github-bot]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@cipolleschi merged this pull request in 89f93dc.

[react-native-bot]

This pull request was successfully merged by @zhongwuzw in 89f93dc

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