Set default content to an empty array instead of nil

Follow-up to https://github.com/modelcontextprotocol/ruby-sdk/pull/147.

According to the specification schema, `content` is not optional and therefore cannot be omitted:

```typescript
interface CallToolResult {
  _meta?: { [key: string]: unknown };
  content: ContentBlock[];
  isError?: boolean;
  structuredContent?: { [key: string]: unknown };
  [key: string]: unknown;
}
```

https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolresult

Instead of `nil`, an empty array is set as the default value.

There may be a better value for `content`, but at the very least it should not be missing.

lib/mcp/tool/response.rb

@@ -13,7 +13,7 @@ module MCP
           error = deprecated_error
         end
 
-        @content = content
+        @content = content || []
         @error = error
         @structured_content = structured_content
       end