Avoid dropping null byte in format string

[user202729]

Consider

fmt::format("a\0{}");
fmt::format("{}\0a");

Both ought to raise a compile-time error because of missing format argument, but before this change, only the second one will. This is because previously s instead of str was passed in the checking function, which uses the following constructor overload

#if FMT_GCC_VERSION
  FMT_ALWAYS_INLINE
#endif
  FMT_CONSTEXPR basic_string_view(const Char* s) : data_(s) {
#if FMT_HAS_BUILTIN(__builtin_strlen) || FMT_GCC_VERSION || FMT_CLANG_VERSION
    if (std::is_same<Char, char>::value && !detail::is_constant_evaluated()) {
      size_ = __builtin_strlen(detail::narrow(s));  // strlen is not constexpr.
      return;
    }
#endif
    size_t len = 0;
    while (*s++) ++len;
    size_ = len;
  }

and drop everything after the null byte.

Also simplify the other constructor since str's type is already string_view.