GraphQL rake task handles missing error path

What does this MR do and why?

In GraphQL query validation rake task, sometimes path is nil which aborts the task. Following the upstream validation, it seems path might be missing if there's a timeout 👉 https://github.com/rmosolgo/graphql-ruby/blob/master/spec/graphql/static_validation/validator_spec.rb#L176

This MR handles path when it is nil so we can see when this occurs and can further debug.

Job failure: https://gitlab.com/gitlab-org/gitlab/-/jobs/3942725978#L1132

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #396784