Update NdjsonPipeline#transform to return if data is nil

What does this MR do and why?

⚠ this feature is behind bulk_import_projects feature flag that is currently off by default (and disabled on .com).

This MR fixes a bug where an exception would be raised in NdjsonPipeline#transform because of incoming data being nil. It is used in a GitLab Migration feature https://docs.gitlab.com/ee/user/group/import/ that imports groups and projects with one click.

For example, when importing project does not have push rule, then the contents of downloaded push_rule.ndjson file is nil, which is being read from file and converted into [nil, 0] during extraction, which raises exception NoMethodError: undefined method except for nil here: http://gitlab.com/gitlab-org/gitlab/blob/6e39022e6514273e2f34a764b6db51af3f844a87/lib/gitlab/import_export/base/relation_factory.rb#L51-L51

This MR makes sure that if no data is exported from source, there is no need to raise an exception, just return early and do nothing.

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How to set up and validate locally

1. Feature.enable(:bulk_import).
2. Feature.enable(:bulk_import_projects).
3. Create a top level group and project in it. Do not specify any push rules.
4. Go to /groups/new#import-group-pane page and enter instance url and access token (needs to be api & read_repository scope).
5. Select newly created group and click Import.
6. Wait for Group import to complete and verify bulk_import_failures table. There should be no error coming from PushRulePipeline.

MR acceptance checklist

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