Serde Issue TableMetadata + default values for partition_specs and sort_orders

[marvinlanhenke]

...while working on the HMS catalog, I noticed missing fields (properties & snapshots) in the metadata.json when serialized.

Currently we check if e.g. properties is_empty() and return None - due to the serde option flag None will be skipped.
This causes downstream errors, since we need at least an empty object - for example "properties":{}
https://github.com/apache/iceberg-rust/blob/main/crates/iceberg/src/spec/table_metadata.rs#L730-L737

Another issue I had are the default values for partition_specs and sort_orders.
The new TableMetadataBuilder does not handle them for now, however we need different default values - for example:
"partition_specs: [{spec-id: 0, fields []}]" otherwise I get downstream errors as well.
https://github.com/apache/iceberg-rust/blob/main/crates/iceberg/src/spec/table_metadata.rs#L328-L335