feature: Add type predicates to isEmpty and isNotEmpty functions

[pemakhov]

To use isEmpty or isNotEmpty in type guards these functions should return type predicates.

In the following example there will be typescript error in the line return a + b even though we have previously checked "a" and "b" were not empty.

function add(a?: number, b?: number) {
  if (isEmpty(a) || isEmpty(b)) {
    throw new Error('a and b must be provided');
  }

  return a + b; // 'a' is possibly undefined
                // 'b' is possibly undefined
}

I propose to alter isEmpty and isNotEmpty to return type predicates.