zstring_span: fix for Expects, simplify functions

[beinhaerter]

Fixes for a few issues recognized while answering https://stackoverflow.com/questions/56874532/using-gslzstring-view-with-c-apis.

• s[s.size() - 1] is wrong for empty s, so Expects(s.size() > 0)
• no hard coded '\0'but value_type{}
• hard code empty() to return false
• simplify as_string_span: can never be empty
• clarify comment on ensure_z

[JordanMaples]

Maintainers' call: The change looks good but would you mind rebasing this so I can merge it?

[beinhaerter]

@JordanMaples Did a rebase. When looking again at the code I am am (again) scratching my head. I did not change the result of empty, but is empty exactly what it should be? When a client of GSL has a basic_zstring_span s and calls if (s.empty()) what did he mean to do? Surely not getting a constant false. And probably not trying to check if the basic_zstring_span does not contain even a terminating null byte - because that is what the class is about and is checking. He probably meant to check if the underlying string is empty. So in my opinion empty should either return span_.size() > 1 or be removed. Such a behaviour change should be addressed in a later PR.

[JordanMaples]

@beinhaerter You're right, empty in its current state doesn't really make much sense. We can definitely address empty in a later PR though. I'll merge the changes now and I'll let you know what the other maintainers think next week after our next meeting. Thanks!