fix panic in Velero analyzer when there's no Velero deployment found

[nvanthao]

Description, Motivation and Context

#1366 introduces a panic when there's no Velero Deployment found.

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x58 pc=0x2f71931]

goroutine 1 [running]:
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End.func1()
	/home/runner/go/pkg/mod/go.opentelemetry.io/otel/[email protected]/trace/span.go:405 +0x25
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End(0xc000ee8480, {0x0, 0x0, 0x106a36b?})
	/home/runner/go/pkg/mod/go.opentelemetry.io/otel/[email protected]/trace/span.go:443 +0xa3b
panic({0x3301020?, 0x55ce110?})
	/opt/hostedtoolcache/go/1.21.6/x64/src/runtime/panic.go:914 +0x21f
github.com/replicatedhq/troubleshoot/pkg/analyze.getVeleroVersion({0x5660860, 0x0, 0x0}, 0xc000f29b60)
	/home/runner/work/troubleshoot/troubleshoot/pkg/analyze/velero.go:669 +0x1f1

Checklist

• New and existing tests pass locally with introduced changes.
• Tests for the changes have been added (for bug fixes / features)
• The commit message(s) are informative and highlight any breaking changes
• Any documentation required has been added/updated. For changes to https://troubleshoot.sh/ create a PR here

Does this PR introduce a breaking change?

• Yes
• No