DPL-812.2: repair undefined method 'value' for nil:NilClass

[StephenHulme]

Closes #1433

Changes proposed in this pull request

• Fixes the bug below by using the Safe Navigation Operator.

  An ActionView::Template::Error occurred in exports#show:
  
  undefined method `value' for nil:NilClass
  app/views/exports/pbmc_bank_tubes_content_report.csv.erb:71
  

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[codeclimate]

Code Climate has analyzed commit 495c056 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 90.0% (0.0% change).

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