`null` value always appended to `enum` array when transforming nullable enum properties

[Drew-Kimberly]

Current Behavior

A null value is always appended to the enum array when transforming nullable enum properties. This results in an invalid JSON Schema document when the OAS already includes null in the array (valid OAS)

Expected Behavior

null is not appended when the enum array already includes it.

Possible Workaround/Solution

Check the elements of the array before appending the null value

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I will PR a fix