Fix fastAddProperties to properly nullify style props (#30334)

## Summary

This PR fixes the `fastAddProperties` function. Now it nullifies a prop
if it was defined in one of the items of a style array, but then set to
`undefined` or `null` in one of the subsequent items. E.g. `style:
[{top: 0}, {top: undefined}]` should evaluate to `{top: null}`. Also
added a test case for that.

## How did you test this change?
```
yarn test packages/react-native-renderer -r=xplat --variant=false
yarn test packages/react-native-renderer -r=xplat --variant=true
yarn flow native
```

packages/react-native-renderer/src/ReactNativeAttributePayloadFabric.js

@@ -464,10 +464,6 @@ function fastAddProperties(
   for (const propKey in props) {
     const prop = props[propKey];
 
-    if (prop === undefined) {
-      continue;
-    }
-
     const attributeConfig = ((validAttributes[
       propKey
     ]: any): AttributeConfiguration);
@@ -478,7 +474,14 @@ function fastAddProperties(
 
     let newValue;
 
-    if (typeof prop === 'function') {
+    if (prop === undefined) {
+      // Discard the prop if it was previously defined.
+      if (payload && payload[propKey] !== undefined) {
+        newValue = null;
+      } else {
+        continue;
+      }
+    } else if (typeof prop === 'function') {
       // A function prop. It represents an event handler. Pass it to native as 'true'.
       newValue = true;
     } else if (typeof attributeConfig !== 'object') {

packages/react-native-renderer/src/__tests__/ReactNativeAttributePayloadFabric-test.internal.js

@@ -60,7 +60,16 @@ describe('ReactNativeAttributePayloadFabric.create', () => {
     });
   });
 
-  it('should ignore fields that are set to undefined', () => {
+  it('should nullify previously defined style prop that is subsequently set to null or undefined', () => {
+    expect(
+      create({style: [{a: 0}, {a: undefined}]}, {style: {a: true}}),
+    ).toEqual({a: null});
+    expect(create({style: [{a: 0}, {a: null}]}, {style: {a: true}})).toEqual({
+      a: null,
+    });
+  });
+
+  it('should ignore non-style fields that are set to undefined', () => {
     expect(create({}, {a: true})).toEqual(null);
     expect(create({a: undefined}, {a: true})).toEqual(null);
     expect(create({a: undefined, b: undefined}, {a: true, b: true})).toEqual(