Prevent construction/conversion from nullptr

[mbs-c]

When introducing string_view into an old codebase, it is quite easy to accidentally leave nonsensical assignments from or comparisons to nullptr in your code. Adding a deleted constructor as follows allows developers to easily identify the offending assignments and comparisons:

#if nssv_CPP11_OR_GREATER
    nssv_constexpr basic_string_view( std::nullptr_t ) nssv_noexcept = delete;
#endif

C++23 will also disallow constructing std::basic_string_view from nullptr: cplusplus/papers#887

[martinmoene]

Ah, thank you for the pointer Moritz @mbs-c, I wasn't yet aware of this.

I'll add the proposed constructor.