I just found that xcode/clang does not allow to construct a std::string_view from a char* nullptr. I would expect that this sets size() to 0 and returns a null for data(), and this is how also how gcc 7.2 on my Ubuntu Linux box implemented it.
I checked http://en.cppreference.com/ but couldn't find any comment about this, so what is the standard saying?
Well, yes and no. You cannot pass a null pointer to std::string_view's constructors, because that will violate their preconditions; see constructor 2 and 3 in [string.view.cons].
But std::string_view's default constructor sets size to zero and null for its data, so you will have to use that constructor instead, or use this workaround/trick:
std::string_view().swap(your_string);