fix: Improve null handling in code block and syntax highlighting

src/renderer/src/context/SyntaxHighlighterProvider.tsx

@@ -60,7 +60,7 @@ export const SyntaxHighlighterProvider: React.FC<PropsWithChildren> = ({ childre
 
     const mappedLanguage = languageMap[language] || language
 
-    code = code.trimEnd()
+    code = code?.trimEnd() ?? ''
     const escapedCode = code?.replace(/[<>]/g, (char) => ({ '<': '&lt;', '>': '&gt;' })[char]!)
 
     try {

src/renderer/src/pages/home/Markdown/CodeBlock.tsx

@@ -154,6 +154,7 @@ const CopyButton: React.FC<{ text: string; style?: React.CSSProperties }> = ({ t
   const { t } = useTranslation()
 
   const onCopy = () => {
+    if (!text) return
     navigator.clipboard.writeText(text)
     window.message.success({ content: t('message.copied'), key: 'copy-code' })
     setCopied(true)