Change empty classes array to one line format

What does this MR do and why?

There has been some unintended churn on the file db/docs/verification_codes.yml recently. Starting with 8837ba86 and then d4e16782 and then be1bffa7

I think this churn is because different versions of libyaml dump multiline empty lists differently and users are getting different results depending on whether they have a version >= 0.2.5 of libyaml.

Changing the empty classes key in this file from:

classes:
-

to

classes: []

Will stop this churn from occurring and will match all of the other empty classes lists in the db/docs directory, e.g here, here, etc ...

This basically reverts the declaration back to how it was before the first of the churn commits listed above.

---

The multi line format is parsed as an Array with a single nil value, e.g.:

puts YAML.safe_load("---\nclasses:\n-\n").inspect
{"classes"=>[nil]}

This structure is dumped slightly differently by different versions of libyaml and has lead to some unintended churn on this file.

# libyaml < 0.2.5 - note the additional whitespace
puts ({classes: [nil]}.to_yaml.inspect)
"---\n:classes:\n- \n"

# libyaml >= 0.2.5
puts ({classes: [nil]}.to_yaml.inspect)
"---\n:classes:\n-\n"

This change reverts the classes empty array back to a single line representation to match the other files in db/docs which have an empty classes array:

puts YAML.safe_load("---\nclasses: []\n").inspect
{"classes"=>[]}