two-fer: Consistency of the empty string

In some languages we are able to define functions with the same name and different aritys. Those languages will automatically treat `hello()` different to `hello("")`.

Other languages do not have this luxury or do allow to use default values for arguments.

So in the C track I realized, the empty string is used to signal “nothing given”, while `NULL` might be more appropriate.

Other languages have similar (perhaps more noisy) ways to differ between an empty string and “nothing given” by `Maybe` or `Option` style types.

Therefore I wanted to start a discussion about treatement of the empty string.
1. Treat it as if we had called without a name, so we create `"Hello, World!"` as output
2. Treat it as proper name and return `"Hello, !"`
3. Leave it open to language maintainers

I tried to look up at least the tracks I already solved `hello-world` in:
- Different functions by arity: Erlang, Elixir
- empty string is nothing: C
- empty string is proper name: Rust (`Option`-type)
- testsuite is unclear about the meaning of the empty string: Scala (default mechanisms used based on NULL-references), Crystal (as Scala), Scheme (as Scala), PHP (default mechanisms based on PHP stuff I deny to delve into)

Especially in the last group, it mostly depends on the student how he implements the handling of the default value. I've seen solutions in such exercises that check default `name` to `NULL` (or similar) or `""` and then do a conditional reassignment in the functions body (which in my opinion defeats the purpose of default values for argument).

Especially this wild growing stuff is in my opinion dangerous. In many tracks there are not many people who actually review, and even after `hello-world` is not hidden any more, many reviewers seem to just skip it.