Snapshot summary totals appear to be calculated incorrectly when previous totals don't exist

[dannycjones]

Apache Iceberg Rust version

None

Describe the bug

When calculating totals, we initialize the previous as 0 if not set. Given these summaries are optional, we may incorrectly calculate the totals where files exist but don't have total summaries set.

iceberg-rust/crates/iceberg/src/spec/snapshot_summary.rs

Lines 486 to 491 in 130ea0f

	let previous_total = previous_summary.map_or(0, |previous_summary| {
	previous_summary
	.additional_properties
	.get(total_property)
	.map_or(0, |value| value.parse::<u64>().unwrap())
	});

To Reproduce

No response

Expected behavior

I expect that we should skip calculating the totals when we do not have a previous value to reference. This does mean some other process would need to calculate the totals outside of this commit in order to introduce totals to the snapshot summaries.

Willingness to contribute

I can contribute a fix for this bug independently

[dannycjones]

@CTTY does this sound like a real bug? I am happy to make this change if so (and I'd like to include in #2527).