nil error

I merged a PR supposedly adding structured-acceptance-test support to my project, but now I get a nil error:

```console
$ cucumber
Feature: Run example tests

Scenario: Running example tests # features/run_example_tests.feature:2
/Users/andrew/.gem/ruby/2.3.7/gems/structured-acceptance-test-0.0.6/lib/finding.rb:57:in `block in categories=': undefined method `include?' for nil:NilClass (NoMethodError)
from /Users/andrew/.gem/ruby/2.3.7/gems/structured-acceptance-test-0.0.6/lib/finding.rb:55:in `each'
from /Users/andrew/.gem/ruby/2.3.7/gems/structured-acceptance-test-0.0.6/lib/finding.rb:55:in `categories='
from /Users/andrew/go/src/github.com/mcandre/cowl/lib/cowl.rb:88:in `to_finding'
from /Users/andrew/go/src/github.com/mcandre/cowl/lib/cowl.rb:152:in `block in check'
from /Users/andrew/go/src/github.com/mcandre/cowl/lib/cowl.rb:151:in `each'
from /Users/andrew/go/src/github.com/mcandre/cowl/lib/cowl.rb:151:in `check'
from /Users/andrew/go/src/github.com/mcandre/cowl/bin/cowl:72:in `block in main'
from /Users/andrew/go/src/github.com/mcandre/cowl/bin/cowl:58:in `each'
from /Users/andrew/go/src/github.com/mcandre/cowl/bin/cowl:58:in `main'
from /Users/andrew/go/src/github.com/mcandre/cowl/bin/cowl:100:in `'
from /Users/andrew/.gem/ruby/2.3.7/bin/cowl:23:in `load'
from /Users/andrew/.gem/ruby/2.3.7/bin/cowl:23:in `'
Given the program has finished # features/step_definitions/steps.rb:1
Then the output is correct for each test # features/step_definitions/steps.rb:12

expected: true
got: false

(compared using ==)
(RSpec::Expectations::ExpectationNotMetError)
features/run_example_tests.feature:4:in `Then the output is correct for each test'

Failing Scenarios:
cucumber features/run_example_tests.feature:2 # Scenario: Running example tests

1 scenario (1 failed)
2 steps (1 failed, 1 passed)
```