Optimize DynamoDBMapper DELETE without empty expected maps

[RanVaknin]

Summary

• Avoid populating DeleteItemRequest.expected when the mapper has no version assertion or user-provided expected conditions.
• Preserve versioned deletes, condition expressions, expected-value expressions, and conditional operators.
• Update the golden request fixture for the simple delete path: the request now omits Expected instead of serializing "Expected": {}.

This removes construction and defensive copying of an empty map from the common delete path.

Compatibility

DynamoDB service behavior is unchanged. Code that inspects the low-level request through an interceptor or mock can observe expected() == null instead of an empty map for an unconditional delete.

Testing

• All 620 DynamoDB Mapper tests passed on the candidate host.
• Golden request coverage verifies unconditional, versioned, condition-expression, and expected-expression deletes.

Performance

The baseline and candidate ran on separate, equivalently configured c6g.metal hosts using JMH 3-3-2, -prof gc, deterministic stub clients, and the complete 24-case Mapper v2 matrix.

DELETE payload	Throughput	Allocation
TINY	+6.72%	-14.86% B/op
SMALL	+6.83%	-14.86% B/op
HUGE	+11.18%	-14.86% B/op
HUGE_FLAT	+13.43%	-14.86% B/op

The full matrix initially showed GET/HUGE_FLAT at -3.08%. A focused 5-5-2 rerun measured 174,830 ops/s for the baseline and 176,162 ops/s for the candidate, a statistically insignificant +0.76%, confirming it was unrelated benchmark noise.

Complete throughput matrix

Benchmark	Baseline ops/s	Candidate ops/s	Change
DELETE / HUGE_FLAT	3,876,820	4,397,470	+13.43%
SCAN / TINY	990,350	1,102,703	+11.35%
DELETE / HUGE	3,600,851	4,003,284	+11.18%
DELETE / SMALL	3,794,745	4,054,061	+6.83%
DELETE / TINY	3,765,409	4,018,593	+6.72%
UPDATE / HUGE_FLAT	58,560	60,889	+3.98%
GET / HUGE_FLAT	180,157	174,600	-3.08%
UPDATE / SMALL	672,137	691,341	+2.86%
UPDATE / TINY	1,337,973	1,375,522	+2.81%
PUT / HUGE_FLAT	93,199	95,419	+2.38%
SCAN / HUGE_FLAT	57,666	58,731	+1.85%
GET / TINY	2,114,996	2,100,916	-0.67%
PUT / TINY	2,077,859	2,065,919	-0.57%
GET / HUGE	145,592	144,870	-0.50%
UPDATE / HUGE	85,982	85,562	-0.49%
QUERY / HUGE	46,975	47,131	+0.33%
PUT / HUGE	93,210	92,961	-0.27%
SCAN / HUGE	49,205	49,292	+0.18%
PUT / SMALL	1,058,703	1,056,992	-0.16%
QUERY / SMALL	419,135	419,679	+0.13%
QUERY / TINY	721,027	721,929	+0.13%
GET / SMALL	1,236,414	1,235,489	-0.07%
SCAN / SMALL	532,499	532,799	+0.06%
QUERY / HUGE_FLAT	56,678	56,654	-0.04%