parsing of JSONschema: null enum value is thrown away

[rienafairefr]

issue coming from downstream (swagger-parser and openapi-generator)
A value of null in a JSON schema enum should be perfectly valid if I'm reading the spec correctly, but it's not added to the enum value when parsing:

schema:
  enum: [A, B, C, null]
  type: string

results in the schema having getEnum() of ["A", "B", "C"] instead of ["A", "B", "C", null].
This bug was introduced by #2510 @frantuma. A cast value of null might be that the passed value does not cast to the intended type, but it might also mean the value is actually null, and this case shouldn't be discarded.