refactor: Funnel Children 요소가 없는 경우, prev, next 함수의 방어코드 작성

src/components/Funnel/Funnel.tsx

@@ -1,26 +1,31 @@
-import React, { createContext, useState, useMemo, PropsWithChildren, useContext } from 'react';
+import { createContext, useState, useMemo, PropsWithChildren, useContext, Children } from 'react';
 
 type FunnelContextType = {
   currentIndex: number;
   goToNext: () => void;
   goToPrev: () => void;
 };
 
-export const FunnelContext = createContext<FunnelContextType | undefined>(undefined);
+export const FunnelContext = createContext<FunnelContextType | null>(null);
 
 type FunnelProps = PropsWithChildren<{
   startIndex?: number;
 }>;
-
 export const Funnel = ({ children, startIndex }: FunnelProps) => {
   const [currentIndex, setCurrentIndex] = useState(startIndex ?? 0);
-  const lastIndex = React.Children.count(children) - 1;
+  const lastIndex = Children.count(children) - 1;
 
   const goToNext = () => {
+    if (lastIndex < 0) {
+      return;
+    }
     setCurrentIndex((prevIndex) => (prevIndex >= lastIndex ? 0 : prevIndex + 1));
   };
 
   const goToPrev = () => {
+    if (lastIndex < 0) {
+      return;
+    }
     setCurrentIndex((prevIndex) => (prevIndex <= 0 ? lastIndex : prevIndex - 1));
   };
 
@@ -37,7 +42,7 @@ const FunnelStep = ({ index, children }: FunnelStepProps) => {
 
 export const useFunnel = () => {
   const context = useContext(FunnelContext);
-  if (!context) {
+  if (context === null) {
     throw new Error('useFunnel은 Funnel.Root 하위에서 사용되어야 합니다.');
   }
   return context;