Add new clang-tidy check for string_view(nullptr)

Checks for various ways that the `const CharT*` constructor of `std::basic_string_view` can be passed a null argument and replaces them with the default constructor in most cases. For the comparison operators, braced initializer list does not compile so instead a call to `.empty()` or the empty string literal are used, where appropriate.

This prevents code from invoking behavior which is unconditionally undefined. The single-argument `const CharT*` constructor does not check for the null case before dereferencing its input. The standard is slated to add an explicitly-deleted overload to catch some of these cases: wg21.link/p2166

https://reviews.llvm.org/D114823 is a companion change to prevent duplicate warnings from the `bugprone-string-constructor` check.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D113148