Set default content to an empty array instead of nil

Follow-up to #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.

Author: koic

lib/mcp/tool/response.rb

@@ -7,7 +7,7 @@ class Response
 
       attr_reader :content, :structured_content
 
-      def initialize(content = nil, deprecated_error = NOT_GIVEN, error: false, structured_content: nil)
+      def initialize(content = [], deprecated_error = NOT_GIVEN, error: false, structured_content: nil)
         if deprecated_error != NOT_GIVEN
           warn("Passing `error` with the 2nd argument of `Response.new` is deprecated. Use keyword argument like `Response.new(content, error: error)` instead.", uplevel: 1)
           error = deprecated_error

test/mcp/tool/response_test.rb

@@ -106,8 +106,8 @@ class ResponseTest < ActiveSupport::TestCase
         response = Response.new(structured_content: structured_content)
         actual = response.to_h
 
-        assert_equal [:isError, :structuredContent], actual.keys
-        assert_nil actual[:content]
+        assert_equal [:content, :isError, :structuredContent], actual.keys
+        assert_empty actual[:content]
         assert_equal structured_content, actual[:structuredContent]
         refute actual[:isError]
       end