isNull args as a list

[Ariana-B]

Resolves #91

Accepts isNull args as both arrays and objects for consistency with not implementation. However it's worth noting that the comment regarding the standard being ambiguous might be outdated as the standard seems relatively clear to me:

notExpression:
      type: object
      required:
        - op
        - args
      properties:
        op:
          type: string
          enum:
            - not
        args:
          type: array
          minItems: 1
          maxItems: 1
          items:
            $ref: '#/components/schemas/booleanExpression'

Likewise:

isNullPredicate:
      type: object
      required:
        - op
        - args
      properties:
        op:
          type: string
          enum:
            - isNull
        args:
          $ref: '#/components/schemas/isNullOperand'
    isNullOperand:
      type: array
      minItems: 1
      maxItems: 1
      items:
        oneOf:
          - $ref: '#/components/schemas/characterExpression'
          - $ref: '#/components/schemas/numericExpression'
          - $ref: '#/components/schemas/temporalExpression'
          - $ref: '#/components/schemas/booleanExpression'
          - $ref: '#/components/schemas/geomExpression'

[Ariana-B]

@constantinius