Add warning for style animations missing initial values (#3947)

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## Summary
Adds a warning for style animations missing initial values.

<!-- Explain the motivation for this PR. Include "Fixes #<number>" if
applicable. -->

Example/src/LayoutReanimation/Modal.tsx

@@ -46,7 +46,15 @@ function AnimatedView() {
     'worklet';
 
     return {
-      initialValues: {},
+      initialValues: {
+        transform: [
+          { translateY: targetValues.currentHeight / 2 },
+          { perspective: 500 },
+          { rotateX: '0deg' },
+          { translateY: -targetValues.currentHeight / 2 },
+          { translateY: 0 },
+        ],
+      },
       animations: {
         transform: [
           { translateY: withTiming(targetValues.currentHeight / 2) },

src/reanimated2/animation/styleAnimation.ts

@@ -46,7 +46,7 @@ export function setPath<T>(
   for (let i = 0; i < keys.length - 1; i++) {
     // creates entry if there isn't one
     currObj = currObj as { [key: string]: NestedObjectValues<T> };
-    if (!currObj[keys[i]]) {
+    if (!(keys[i] in currObj)) {
       // if next key is a number create an array
       if (typeof keys[i + 1] === 'number') {
         currObj[keys[i]] = [];
@@ -178,6 +178,13 @@ export function withStyleAnimation(
           if (prevAnimation && !prevVal) {
             prevVal = prevAnimation.current;
           }
+          if (prevVal === undefined) {
+            console.warn(
+              `Initial values for animation are missing for property ${currentEntry.path.join(
+                '.'
+              )}`
+            );
+          }
           setPath(animation.current, currentEntry.path, prevVal);
           let currentAnimation: AnimationObject;
           if (