fix(pier): a tools/call without arguments or with null arguments is an empty object

- MCP's tools/call arguments are optional; pier validated a JSON null
  against the input schema and refused every call of a tool without
  required input that an agent sent without arguments
- found by plan 19.7-05's TestDispatch

modules/pier/dispatch.go

@@ -69,7 +69,9 @@ func (d *dispatcher) call(ctx context.Context, id Identity, info callInfo, t *to
 }
 
 func (d *dispatcher) run(ctx context.Context, id Identity, info callInfo, t *tool, raw json.RawMessage) (*mcp.CallToolResult, int) {
-	if len(bytes.TrimSpace(raw)) == 0 {
+	// tools/call may omit arguments (or send null) for a tool without
+	// required input; both mean an empty object.
+	if trimmed := bytes.TrimSpace(raw); len(trimmed) == 0 || bytes.Equal(trimmed, []byte("null")) {
 		raw = json.RawMessage("{}")
 	}
 	var parts map[string]json.RawMessage