fix(search): add null check for canvas elements in fulltext search

添加对 Canvas 笔记 elements 字段的空值检查，防止当 elements 为 null 或非数组时搜索功能报错。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/server/src/services/search/expressions/note_content_fulltext.ts

@@ -317,11 +317,16 @@ class NoteContentFulltextExp extends Expression {
 
             let canvasContent = JSON.parse(content);
             const elements: Element[] = canvasContent.elements;
+
+            if (elements && Array.isArray(elements)) {
                 const texts = elements
                     .filter((element: Element) => element.type === "text" && element.text) // Filter for 'text' type elements with a 'text' property
                     .map((element: Element) => element.text!); // Use `!` to assert `text` is defined after filtering
 
                 content = normalize(texts.toString());
+            } else {
+                content = "";
+            }
         }
 
         return content.trim();