Add missing null-check to findHostInstanceFastPath (#6884)

## Summary

This PR adds a missing null check for certain components that do not
contain the `_componentRef` field. This can happen for example when you
uses `useAnimatedRef` with FlashList.

## Test plan

Use FlashList as a animated component

packages/react-native-reanimated/src/platform-specific/findHostInstance.ts

@@ -18,7 +18,10 @@ type HostInstancePaper = {
 
 export type HostInstance = HostInstanceFabric & HostInstancePaper;
 
-function findHostInstanceFastPath(maybeNativeRef: HostInstance) {
+function findHostInstanceFastPath(maybeNativeRef: HostInstance | undefined) {
+  if (!maybeNativeRef) {
+    return undefined;
+  }
   if (
     maybeNativeRef.__internalInstanceHandle &&
     maybeNativeRef.__nativeTag &&