GitHub Import: Skip label link creation if issue not found

What does this MR do and why?

Corresponding issue: ActiveRecord::RecordInvalid: Validation failed:... (#389266 - closed)

There was a lot of occurrences of error in GitHub Import

ActiveRecord::RecordInvalid: Validation failed: Target can't be blank
  from activerecord (6.1.6.1) lib/active_record/validations.rb:80:in `raise_validation_error'
  from activemodel (6.1.6.1) lib/active_model/validations.rb:383:in `validate!'
  from app/models/concerns/bulk_insert_safe.rb:171:in `block in _bulk_insert_item_attributes'
  from app/models/concerns/bulk_insert_safe.rb:170:in `map'
  from app/models/concerns/bulk_insert_safe.rb:170:in `_bulk_insert_item_attributes'
  from app/models/concerns/bulk_insert_safe.rb:158:in `block (2 levels) in _bulk_insert_all!'
  from app/models/concerns/bulk_insert_safe.rb:157:in `each'
  from app/models/concerns/bulk_insert_safe.rb:157:in `each_slice'
  from app/models/concerns/bulk_insert_safe.rb:157:in `each'
  from app/models/concerns/bulk_insert_safe.rb:157:in `flat_map'
  from app/models/concerns/bulk_insert_safe.rb:157:in `block in _bulk_insert_all!' 
  from app/models/concerns/bulk_insert_safe.rb:156:in `_bulk_insert_all!'
  from app/models/concerns/bulk_insert_safe.rb:91:in `bulk_insert!'
  from lib/gitlab/github_import/importer/label_links_importer.rb:43:in `create_labels'
  from lib/gitlab/github_import/importer/label_links_importer.rb:20:in `execute'
  from lib/gitlab/github_import/importer/issue_and_label_links_importer.rb:20:in `execute'
  from app/workers/concerns/gitlab/github_import/object_importer.rb:38:in `import'
  from app/workers/concerns/gitlab/github_import/rescheduling_methods.rb:29:in `try_import'
  from app/workers/concerns/gitlab/github_import/rescheduling_methods.rb:18:in `perform'

The issue is hard to reproduce. I tried various imports and didn't hit this error unless manually mimic the behavior. The only possible scenarios that I can see is when the target is blank:

• The project has been deleted since scheduling issue import jobs. In this case, it silently skips issue creation which results in a blank target_id. https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/github_import/importer/issue_importer.rb#L68
• The issue failed to be created due to validation errors, which is also results on blank target_id
• The Redis cache was cleared (don't think it is happening)

Bearing that in mind it is logical to skip label_link creation if the issue failed to be created for any reason.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.