refactor(search): simplify null check and use join for text concatenation

根据代码审查建议优化代码：
- 移除多余的 `elements &&` 检查，因为 Array.isArray() 本身可处理 null/undefined
- 使用 `join(" ")` 替代 `toString()` 以确保文本元素用空格分隔，更适合全文搜索
- 移除显式类型声明，让 TypeScript 自动推断

🤖 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

@@ -315,15 +315,15 @@ class NoteContentFulltextExp extends Expression {
                 [key: string]: any; // Other properties that may exist
             }
 
-            let canvasContent = JSON.parse(content);
-            const elements: Element[] = canvasContent.elements;
+            const canvasContent = JSON.parse(content);
+            const elements = canvasContent.elements;
 
-            if (elements && Array.isArray(elements)) {
+            if (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());
+                content = normalize(texts.join(" "));
             } else {
                 content = "";
             }