Fix exists operator for boolean question types

[vesnushka]

Exists operator is not applied for boolean question types

item:
  - text: Question 1
    type: boolean
    linkId: question1
  - text: Question 2
    linkId: question2
    enableWhen:
      - answer:
          boolean: false
        operator: exists
        question: question1

[projkov]

Unfortunately, this problem exists not only for the boolean type of question. It also has problems with integers and decimals.
It's because _.isEmpty is not the correct function to work with it.

Checks if value is an empty object, collection, map, or set.

[projkov]

The actual information about the issue is stored here: #63