[FEATURE] support blank parameters of tools

[invisibleyoung]

Thank you for creating a great go mcp SDK.
in protocol module, the declaration of InputSchema is:

type InputSchema struct {
	Type       InputSchemaType      `json:"type"`
	Properties map[string]*Property `json:"properties,omitempty"`
	Required   []string             `json:"required,omitempty"`
}

I used the Client SDK to communicate with LLM, during my development I found that when a remote tool params is blank, the protocol.InputSchema will omit the Properties, the generated JSON string is illegal for OpenAI tool calling json schema.
for now, the blank param schema is:

{
"type": "object"
}

the expected params format which could be accepted by OpenAI api is:

{
"type": "object",
"properties": {}
}

Could you remove the omitempty tag of Promperties?

[jesse-engineer]

Merely removing the omitempty tag does not seem to solve the problem, because this will make

{
"type": "object",
"properties": null
}

[jesse-engineer]

Merely removing the omitempty tag does not seem to solve the problem, because this will make

{
"type": "object",
"properties": null
}

@invisibleyoung You can see if this return can go through the interface of openai.

[jesse-engineer]

#184 I have made some improvements in this PR, please try version v0.2.18 again.

[invisibleyoung]

#184 I have made some improvements in this PR, please try version v0.2.18 again.

Thanks. I'll try it immediately.