fix: support empty object example value for map[string]struct{}

[shindonghwi]

Summary

• Adds support for parsing {} as an empty object in example values
• Fixes the error object is unsupported type in example value {} when using map[string]struct{} with examples

Problem

When using a map[string]struct{} field with swaggertype and example tags:

type Response struct {
    MyField map[string]struct{} `json:"my_field" swaggertype:"object,object" example:"key1:{},key2:{}"`
}

The parser would fail with:

object is unsupported type in example value `{}`

Solution

Added a special case in defineTypeOfExample to handle empty object {} as a valid example value, returning an empty map[string]any{}.

Test plan

• Added unit tests for empty object parsing
• Added unit tests for map[string]struct{} with empty object values
• All existing tests pass

Fixes #2018