When an MCP server (e.g. Python/fastmcp) omits optional schema fields
like required, $defs, and definitions, Jackson deserializes them as
null. This causes failures when callers re-serialize the JsonSchema
with a standard ObjectMapper or iterate over these collections.

Add a compact constructor that defaults null collection fields to
empty immutable collections (List.of(), Map.of()). Also add
@JsonInclude(NON_EMPTY) on collection fields so that empty defaults
are omitted during serialization, preserving wire format compatibility.

Fixes modelcontextprotocol#664

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>