empty properties in JSON Schema for tools is stripped

This is golang/go#76777. It seems to implicate the SDK and/or the jsonschema module.

I tried a couple of fixes (google/jsonschema-go#59, google/jsonschema-go#60) but it didn't help.

To repro: create a tool with empty properties (e.g. https://go-review.git.corp.google.com/c/tools/+/731503/1/gopls/internal/mcp/mcp.go) and call it from the inspector.

/cc @madelinekalil