[libbeat] Input reload: do not log error at debug level if error is nil

[belimawr]

Proposed commit message

see title

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

1. Create the following filebeat.yaml

filebeat.config:
  inputs:
    enabled: true
    path: inputs.d/*.yml
    reload.enabled: true
    reload.period: 5s

output.discard:
  enabled: true

logging:
  to_stderr: true
  level: debug
  selectors:
    - "*"

2. Create the following inputs.d/filestream.yml

- type: filestream
  id: foo
  paths:
    - /tmp/flog.log

Start Filebeat and ensure the following debug log level message does not appear

{
  "@timestamp": "2025-10-31T20:20:32.171-0400",
  "ecs.version": "1.6.0",
  "log.level": "debug",
  "log.logger": "crawler.input.reloader",
  "log.origin": {
    "file.line": 231,
    "file.name": "cfgfile/reload.go",
    "function": "github.com/elastic/beats/v7/libbeat/cfgfile.(*Reloader).Run"
  },
  "message": "error '<nil>' cannot retried. Modify any input file to reload.",
  "service.name": "filebeat"
}

## Related issues
## Use cases
## Screenshots
## Logs

[github-actions]

🤖 GitHub comments

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[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions]

@Mergifyio backport 8.19 9.1 9.2

[mergify]

backport 8.19 9.1 9.2

✅ Backports have been created

• #47453 [8.19](backport #47422) [libbeat] Input reload: do not log error at debug level if error is nil has been created for branch 8.19 but encountered conflicts
• #47451 [9.1](backport #47422) [libbeat] Input reload: do not log error at debug level if error is nil has been created for branch 9.1
• #47452 [9.2](backport #47422) [libbeat] Input reload: do not log error at debug level if error is nil has been created for branch 9.2