fix:withDecay animation not starting if inital value is set to 0

[patrycjakalinska]

Summary

To ensure the animation is correctly initialized and starts as expected we set its current value to undefined. Setting current to 0 breaks the animation.

Fixes #6680.

Test plan

Run code snippet in React Native Paper example 🚀

Code snippet used in issue

import React from 'react';
import { Pressable, Text } from 'react-native';
import Animated, {
  useAnimatedStyle,
  useSharedValue,
  withDecay,
} from 'react-native-reanimated';

export default function EmptyExample() {
  const translateY = useSharedValue(0); 

  const style = useAnimatedStyle(() => ({
    transform: [{ translateY: translateY.value }],
  }));

  return (
    <>
      <Pressable
        style={{ padding: 100, backgroundColor: 'purple' }}
        onPress={() => {
          translateY.value = withDecay({
            velocity: 200,
          });
        }}>
        <Text style={{ fontSize: 50 }}>open</Text>
      </Pressable>
      <Animated.View
        style={[
          {
            backgroundColor: 'blue',
            width: 100,
            height: 100,
          },
          style,
        ]}
      />
    </>
  );
}