Format validation is being performed on empty strings

[eric5694]

This seems to be an issue introduced in #1096. If an openapi property is defined like:

    DateOfBirth:
      description: Date of birth in yyyy-mm-dd format.
      example: "1970-01-01"
      format: date
      type: string
      allowEmptyValue: true

format validation will attempt to be performed on empty strings. This looks like it's because parsePrimitive used to return nil for empty strings, but is now returning the empty string back.