fix(UBSAN): ensure [RCTUITextField validAttributesForMarkedText] Is non null (#2515)

## Summary:

Internally, we build with
[UndefinedBehaviorSanitizer](https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html),
which caught an instance of a method returning nil while marked as non
null. Apple documentation says that method should return an empty array
as a fallback, so let's do that.

## Test Plan:

Tested internally.

Author: Saadnajmi

packages/react-native/Libraries/Text/TextInput/Singleline/RCTUITextField.mm

@@ -161,7 +161,7 @@ - (BOOL)hasMarkedText
 
 - (NSArray<NSAttributedStringKey> *)validAttributesForMarkedText
 {
-	return ((NSTextView *)self.currentEditor).validAttributesForMarkedText;
+	return ((NSTextView *)self.currentEditor).validAttributesForMarkedText ?: @[];
 }
 
 #endif // macOS]