Fix runtime error in isBezierIcon function (#2348)

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## Related Issue

<!-- Please link to issue if one exists -->

<!-- Fixes #0000 -->

- None

## Summary

<!-- Please brief explanation of the changes made -->

아래 함수에서 생길 수 있는 버그를 수정합니다. arg가 null 일 때 typeof arg === 'object' 이기 때문에
런타임 에러가 납니다.

```tsx
export function isBezierIcon(arg: any) {
  return typeof arg === 'object' && arg[BEZIER_ICON_ID] === true
}
```

## Details

<!-- Please elaborate description of the changes -->

- undefined 일 떄는 typeof arg === 'undefined` 이어서 Button 의 leftContent 속성에
아무것도 넣지 않는 경우에는 에러가 뜨지 않고 있었습니다.

### Breaking change? (Yes/No)

<!-- If Yes, please describe the impact and migration path for users -->

- No

## References

<!-- Please list any other resources or points the reviewer should be
aware of -->

-
https://github.com/channel-io/bezier-react/pull/2329/files#diff-7045a70c57f70692f95240783652cbbeb87e13a5528a8c45c69f530e4d62e130

.changeset/wicked-lies-appear.md

@@ -0,0 +1,5 @@
+---
+'@channel.io/bezier-icons': patch
+---
+
+Check if value is nullish in `isBezierIcon` function to block runtime error.

packages/bezier-icons/utils/index.ts

@@ -6,7 +6,7 @@
 const BEZIER_ICON_ID = '__bezier__icon'
 
 export function isBezierIcon(arg: any) {
-  return typeof arg === 'object' && arg[BEZIER_ICON_ID] === true
+  return !!arg && typeof arg === 'object' && arg[BEZIER_ICON_ID] === true
 }
 
 export function createBezierIcon(source: any) {