When tools have no parameters, llm_tool_param_fields_to_json_schema
returns an empty Lua table {}. vim.json.encode serializes empty tables
as JSON arrays [] instead of objects {}, causing Anthropic API to reject
requests with "tools.N.custom.input_schema.properties: Input should be
a valid dictionary" error.
Solution: Use vim.empty_dict() for empty properties to ensure correct
JSON object {} serialization instead of array [].
Tested with Claude Sonnet 4.5 - tools now work correctly without
schema validation errors.
4286ac963a