"_" isn't an empty string. Also, why test for None? None isn't an empty string, so why should it equal "Hello World!"? This solution returns it based off of the fact that only strings are tested. bool("") == False.

• ("_", "Hello, World!"), -> this is wrong according to the specs. You're just expecting things that should not be according to the kata, here.
• (None, "Hello, World!") -> this solution will pass that test (edit: woops, no it will fail on this test. But this input is invalid anyway since there is nothing told about passing None explicitely)

All right add the last two lines to the test!
tests = (
("John", "Hello, John!"),
("aLIce", "Hello, Alice!"),
("", "Hello, World!"),
("_", "Hello, World!"),
(None, "Hello, World!")
)

a space isn't an empty string, so that still complies with the specs.

I believe that if the user put a space.
Then this is an empty string, and the conditions are not met.
We get "Hello,__!"