Fix JsonSchema null collection fields causing serialization failures

[rameshreddy-adutla]

Summary

Fixes #664

When an MCP server (e.g. Python/fastmcp) omits optional schema fields like required, $defs, and definitions, Jackson deserializes them as null. This causes:

• IllegalArgumentException when re-serializing with a standard ObjectMapper
• NullPointerException when iterating over these collections

Changes

McpSchema.JsonSchema (mcp-core)

• Compact constructor: Defaults null collection fields (required, properties, defs, definitions) to empty immutable collections (List.of(), Map.of())
• @JsonInclude(NON_EMPTY) on collection fields: Ensures empty defaults are omitted during serialization, preserving wire format compatibility

Tests (mcp-test)

• testJsonSchemaWithMissingOptionalFields: Verifies deserialization of a minimal schema (no required/$defs/definitions) produces non-null collections and round-trips correctly
• testJsonSchemaConstructorDefaultsForNullCollections: Verifies direct constructor with null args produces empty collections

Before

JsonSchema schema = mapper.readValue("{\"type\":\"object\"}", JsonSchema.class);
schema.required()  // → null
new ObjectMapper().valueToTree(schema)  // → IllegalArgumentException

After

JsonSchema schema = mapper.readValue("{\"type\":\"object\"}", JsonSchema.class);
schema.required()  // → [] (empty list, never null)
new ObjectMapper().valueToTree(schema)  // → works correctly