fix(mockProperty): make sure a falsy value can be assigned to a mock property

[Pmyl]

With this PR the complex property getters will be changed from

get prop() {
  return m["prop"] || m["prop"] = createMock("some_token");
}

to

get prop() {
  return m.hasOwnProperty("prop") ? m["prop"] : m["prop"] = createMock("some_token");
}

In this way even by assigning a null value it won't create the mock.

This will close #207