Return "" instead of null as value of string-based field component

[YevgeniyShunevych]

In Atata v1, when input or content field is empty, null is returned as a value. It is more logical and consistent to return an empty string instead.

Migration to v2.0.0

Take into account to change the assertions like:

SomeInput.Should.BeNull();

to:

SomeInput.Should.BeEmpty();