the backupsize is an intege will make br_300_small_tables fail

[purelind]

Bug Report

Problem Summary: Occasionally, when the size of thebackup-data-size(after-compressed)=501kB is is exactly an integerr, it can result in the test script receiving a null value and exiting unexpectedly.

Translate the text to English, please do not explain any sentences, just translate or leave them as they are.:

[INFO] [collector.go:77] "Database Backup success summary" [total-ranges=300] [ranges-succeed=300] [ranges-failed=0] [backup-checksum=157.211838ms] [backup-fast-checksum=96.622343ms] [backup-total-ranges=300] [total-take=10.427784716s] [BackupTS=443931197293723650] [total-kv=300] [total-kv-size=12.26kB] [average-speed=1.175kB/s] [backup-data-size(after-compressed)=501kB] [Size=500957]

PASS
github.com/pingcap/tidb/br/pkg/lightning/verification	coverage: 0.0% of statements in github.com/pingcap/tidb/br/...
github.com/pingcap/tidb/br/pkg/version	coverage: 25.2% of statements in github.com/pingcap/tidb/br/...

github.com/pingcap/tidb/br/pkg/version/build	coverage: 93.3% of statements in github.com/pingcap/tidb/br/...

The script returned an exit code of 1
Translate the text to English, please do not explain any sentences, just translate or leave them as they are
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