Return an empty String instead of null when creating a mock

[jordigarcl]

trafficstars

See #372 for reference

I believe String values in mocks should return "" by default.

It is a historically 'special' type which has always had some special 'primitive-like' privileges. I assume people expect it to return a nice value as well!

Thank you for submitting a pull request! But first:

• [x] Can you back your code up with tests?
• [x] Keep your commits clean: squash your commits if necessary.

[jordigarcl]

Has this PR been reviewed yet?

[wesalvaro]

What does plain Mockito do?

[wesalvaro]

Plain Mockito returns null.