[8.13](backport #38407) Filebeat gcs input addFiledJobs panic protection

[mergify]

In case obj, err := s.bucket.Object(name).Attrs(ctx) return an error, obj will be probably nil in consequence the code below will panic
objectURI := "gs://" + s.src.BucketName + "/" + obj.Name

Proposed commit message

Filebeat gcs input addFiledJobs panic protection

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• 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
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Logs

Input crashed with: input panic with: runtime error: invalid memory address or nil pointer dereference
goroutine 938 [running]:
runtime/debug.Stack()
	runtime/debug/stack.go:24 +0x65
github.com/elastic/beats/v7/filebeat/input/v2/input-cursor.(*managedInput).runSource.func1()
	github.com/elastic/beats/v7/filebeat/input/v2/input-cursor/input.go:143 +0x5d
panic({0x558d0f92f8a0, 0x558d123846f0})
	runtime/panic.go:884 +0x213
github.com/elastic/beats/v7/x-pack/filebeat/input/gcs.(*scheduler).addFailedJobs(0xc002e6cb40, {0x558d1007a820, 0xc002e720a0}, {0xc0042de240, 0x3, 0x4})
	github.com/elastic/beats/v7/x-pack/filebeat/input/gcs/scheduler.go:238 +0x378
github.com/elastic/beats/v7/x-pack/filebeat/input/gcs.(*scheduler).scheduleOnce(0xc002e6cb40, {0x558d1007a820?, 0xc002e720a0})
	github.com/elastic/beats/v7/x-pack/filebeat/input/gcs/scheduler.go:106 +0x2d3
github.com/elastic/beats/v7/x-pack/filebeat/input/gcs.(*scheduler).schedule(0xc002e6cb40, {0x558d1007a820?, 0xc002e720a0})
	github.com/elastic/beats/v7/x-pack/filebeat/input/gcs/scheduler.go:60 +0x65
github.com/elastic/beats/v7/x-pack/filebeat/input/gcs.(*gcsInput).Run(_, {0xc006f6a490, {0xc002e70000, 0x86}, {{0x558d0eae5cfe, 0x8}, {0x558d0eae5cfe, 0x8}, {0x558d0eadf24e, 0x6}, ...}, ...}, ...)
	github.com/elastic/beats/v7/x-pack/filebeat/input/gcs/input.go:183 +0x885
github.com/elastic/beats/v7/filebeat/input/v2/input-cursor.(*managedInput).runSource(_, {0xc006f6a490, {0xc002e70000, 0x86}, {{0x558d0eae5cfe, 0x8}, {0x558d0eae5cfe, 0x8}, {0x558d0eadf24e, 0x6}, ...}, ...}, ...)
	github.com/elastic/beats/v7/filebeat/input/v2/input-cursor/input.go:168 +0x479
github.com/elastic/beats/v7/filebeat/input/v2/input-cursor.(*managedInput).Run.func1()
	github.com/elastic/beats/v7/filebeat/input/v2/input-cursor/input.go:122 +0x1bf
github.com/elastic/go-concert/unison.(*MultiErrGroup).Go.func1()
	github.com/elastic/go-concert@v0.2.0/unison/multierrgroup.go:42 +0x75
created by github.com/elastic/go-concert/unison.(*MultiErrGroup).Go
	github.com/elastic/go-concert@v0.2.0/unison/multierrgroup.go:40 +0x8d

---

This is an automatic backport of pull request #38407 done by [Mergify](https://mergify.com).

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