iOS Appearance: default to light

Summary:
For iOS < 13, default to "light" appearance, just like in Android (instead of `nil`).

This fixes the following redbox:

{F205818545}

Reviewed By: cpojer

Differential Revision: D17147970

fbshipit-source-id: b2adccd349a0a0ff7668c2f30e93164d23c3eea6

React/CoreModules/RCTAppearance.mm

@@ -32,11 +32,12 @@
     });
 
     traitCollection = traitCollection ?: [UITraitCollection currentTraitCollection];
-    return appearances[@(traitCollection.userInterfaceStyle)] ?: nil;
+    return appearances[@(traitCollection.userInterfaceStyle)] ?: RCTAppearanceColorSchemeLight;
   }
 #endif
 
-  return nil;
+  // Default to light on older OS version - same behavior as Android.
+  return RCTAppearanceColorSchemeLight;
 }
 
 @interface RCTAppearance () <NativeAppearanceSpec>