I recently updated `Cask::DSL` to define instance variables in
`#initialize` to get us closer to resolving a "shape variation"
warning from Ruby. The reason why we continued to receive this warning
after the previous changes is because I overlooked the variables that
are set using `set_unique_stanza`.
The tricky part about those instance variables is that we need to be
able to identify if they've been set. I've handled this by using a
`nil` initial value and updating the `instance_variable_defined?`
condition to check for a non-`nil` value instead. This works for these
variables but it would be a problem if we ever have a DSL method that
accepts a `nil` argument.
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