[Clojure] Handle both types of additional properties when getting type declaration

[njerig]

Fixes #18199.

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[wing328]

lgtm. thanks for the fix

[wing328]

i've filed #18212 to rename the variable to just ap as the generator itself also has a property/field named "additionalProperties"

[wing328]

fyi. filed #18212 to fix the issue in other generators as well.