fix: CSS transitions not working when a prop is added or removed (#9014)

## Summary

I introduced a foolish bug in #8982 while migrating from the
`folly::dynamic` to the `jsi::Value` usage as I forgot to update
`isNull()` checks to `isUndefined()` (undefined values were
automatically converted to nulls in `folly::dynamic` so we had to use
`isNull` before).

## Example recordings

| Before | After |
|-|-|
| <video
src="https://github.com/user-attachments/assets/32947faa-b0f8-4f72-80ee-5df2878caf38"
/> | <video
src="https://github.com/user-attachments/assets/146da9e1-cafc-4751-b03b-434c681ece81"
/> |

It also displayed this warning before:

<img width="1199" height="329" alt="Screenshot 2026-02-26 at 01 04 16"
src="https://github.com/user-attachments/assets/254110b1-3f04-45f5-95d8-ea6c92a852b0"
/>

Author: MatiPl01

apps/fabric-example/ios/Podfile.lock

@@ -51,8 +51,8 @@ void ValueInterpolator::updateKeyframes(jsi::Runtime &rt, const jsi::Value &keyf
 }
 
 bool ValueInterpolator::updateKeyframes(jsi::Runtime &rt, const jsi::Value &fromValue, const jsi::Value &toValue) {
-  auto from = fromValue.isNull() ? defaultStyleValue_ : createValue(rt, fromValue);
-  auto to = toValue.isNull() ? defaultStyleValue_ : createValue(rt, toValue);
+  auto from = fromValue.isUndefined() ? defaultStyleValue_ : createValue(rt, fromValue);
+  auto to = toValue.isUndefined() ? defaultStyleValue_ : createValue(rt, toValue);
 
   const auto equalsReversingAdjustedStartValue = reversingAdjustedStartValue_ && (*to == *reversingAdjustedStartValue_);
   reversingAdjustedStartValue_ = keyframes_.empty() ? from : keyframes_[1].value.value();