Fix crash on extracting subject associations on rspec tests without description

[mbj]

NoMethodError: undefined method `captures' for nil:NilClass
  .gem/ruby/2.7.3/gems/mutant-rspec-0.10.32/lib/mutant/integration/rspec.rb:108:in `parse_metadata'
  .gem/ruby/2.7.3/gems/mutant-rspec-0.10.32/lib/mutant/integration/rspec.rb:93:in `parse_example'
  .gem/ruby/2.7.3/gems/mutant-rspec-0.10.32/lib/mutant/integration/rspec.rb:78:in `block in all_tests_index

Can be triggered from:

RSpec.specify do
  # ...
end