fix: #224

[Flipped199]

Summary

This PR fixes requests for methods without parameters when using a self-hosted Telegram Bot API server.

Previously, rawRequest always created a multipart request body, even when params was nil. That behavior appears to break some self-hosted Bot API implementations for parameterless methods such as getMe.

What changed

• Send a POST request with a nil body when params is nil
• Only create io.Pipe and multipart.Writer when parameters are actually present
• Add a safe isNil helper instead of calling reflect.Value.IsNil() directly on unsupported kinds

Why

This makes parameterless requests behave more like true empty-body HTTP requests instead of empty multipart form uploads.

It also avoids reflection panics for non-nilable value types.

Impact

• Fixes compatibility with self-hosted/custom Telegram Bot API servers
• Keeps existing multipart behavior for requests that actually contain parameters
• Improves safety of nil checks in request building

Related

Fixes #224