fix: do not remove errors with no children

[acheronfail]

I encountered an issue where ajv errors like this:

[
  {
    keyword: 'type',
    dataPath: '/object/type',
    schemaPath: '#/definitions/type/anyOf/0/type',
    params: [Object],
    message: 'should be string'
  },
  {
    keyword: 'type',
    dataPath: '/object/type',
    schemaPath: '#/definitions/type/anyOf/1/type',
    params: [Object],
    message: 'should be array'
  },
  {
    keyword: 'anyOf',
    dataPath: '/object/type',
    schemaPath: '#/definitions/type/anyOf',
    params: {},
    message: 'should match some schema in anyOf'
  }
]

... would return a completely empty error message, since filterRedundantErrors would remove all the errors.

This PR makes sure that filterRedundantErrors doesn't remove the errors if the node doesn't have any children.