[csv ingestor] Fix material.licence when nil

[kennethrioja]

Summary of changes

• Added a method to check for nil licence

Motivation and context

• When using this gsheet for EVERSE (https://docs.google.com/spreadsheets/d/1Ufa4M024k2GeRzP9t64_P9uofXePyGRRf7LOF5vRWTg/edit?gid=448677882#gid=448677882), there was an error in the source test – cause: material.licence = nil

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.

[fbacall]

Better/safer to use minitest's stubbing:

ingestor.stub(:get_column, -> { raise CSV::MalformedCSVError.new('test failure', 22) }) do
  ingestor.read(source.url)
end