Fix: support empty Struct fields in Arrow IPC decoding

[roelentless]

Decoding failed for IPC streams containing empty structs (struct {}) generated by external encoders (e.g. Rust).
According to my research the Arrow spec does not forbid structs without children.

This PR should:

• Fix decoding of empty Struct fields
• Add tests for both IPC formats that use externally generated test data with columns: int32, struct {}, int32

All tests pass

Ran into this with real data; a broader review of empty/null field handling may be worthwhile.

[roelentless]

@jheer if you could have a look? Thx.

[jheer]

Thanks @roelentless!