fix: handle empty read-committed elastic log fetch

[1sonofqiu]

Summary

• return an empty aborted transaction list for empty ElasticLog fetch results under read-committed isolation
• avoid calling ElasticLogSegment.fetchUpperBoundOffset for zero-byte fetch results
• add a regression test for maxLength=0 with includeAbortedTxns=true

Tests

• ./gradlew core:test --tests kafka.log.streamaspect.ElasticLogTest --no-daemon -x checkstyleTest

Notes

• Running the same targeted test without -x checkstyleTest was blocked by an existing untracked file in the working tree: core/src/test/java/kafka/coordinator/group/GroupCoordinatorPartitionTest.java violates MethodLength. That file is not part of this PR.