Fix error for missing fields in dependencies.csv parser

[tiegz]

When a dependencies.csv has a blank field, we were check for emptiness without checking for nilness first:

Before

{:dependencies=>nil, :error_message=>"dependencies.csv: undefined method `empty?' for nil:NilClass", :kind=>"lockfile", :path=>"dependencies.csv", :platform=>"generic", :success=>false}

After

{:dependencies=>nil, :error_message=>"dependencies.csv: missing field 'requirement' on line 1", :kind=>"lockfile", :path=>"dependencies.csv", :platform=>"generic", :success=>false}