Update PING request handler, return empty map instead of empty string

[fengbaicanhe]

change ping request handler, response empty map instead of empty string.

Motivation and Context

MCP Ping Specification
the ping response result field should be empty object:

{
  "jsonrpc": "2.0",
  "id": "123",
  "result": {}
}

How Has This Been Tested?

use mcp official tool npx @modelcontextprotocol/inspector, connect to mcp server and use ping for test.

Breaking Changes

no

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[tzolov]

Than you for catching this @fengbaicanhe

[tzolov]

Updated, rebased and merged at cf0be36